Update pagination buttons to include results count

[JPrevost]

NOTE: you can see the end state logic using the search rails in the timdex tab

Why are these changes being introduced:

• UX design includes showing the number of results that will be shown when clicking the Next or Previous pagination buttons.
• Previous page should always be the same number of results as per_page so it is simpler to say "Previous 20 results" rather than trying to calculate how many results remain before the start index.
• Next page should show the remaining results if fewer than per_page results remain.
• Also introduces a disabled state via a span for the next and previous buttons when there are no more results in that direction.

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/USE-184

How does this address that need:

• Mathed out the new pagination button text requirements
• Updated PaginationHelper methods to calculate remaining results
• Updated PaginationHelper methods to render disabled span when no more results in that direction

Developer

Accessibility

• ANDI or WAVE has been run in accordance to our guide.
• This PR contains no changes to the view layer.
• New issues flagged by ANDI or WAVE have been resolved.
• New issues flagged by ANDI or WAVE have been ticketed (link in the Pull Request details above).
• No new accessibility issues have been flagged.

New ENV

• All new ENV is documented in README.
• All new ENV has been added to Heroku Pipeline, Staging and Prod.
• ENV has not changed.

Approval beyond code review

• UXWS/stakeholder approval has been confirmed.
• UXWS/stakeholder review will be completed retroactively.
• UXWS/stakeholder review is not needed.

Additional context needed to review

E.g., if the PR includes updated dependencies and/or data
migration, or how to confirm the feature is working.

Code Reviewer

Code

• I have confirmed that the code works as intended.
• Any CodeClimate issues have been fixed or confirmed as
  added technical debt.

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  (not just this pull request message).
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[coveralls]

Pull Request Test Coverage Report for Build 19371564204

Details

• 26 of 26 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 97.992%

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Totals
Change from base Build 19342635433:	0.04%
Covered Lines:	927
Relevant Lines:	946

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💛 - Coveralls

[jazairi]

This looks good and works as intended. I'm a bit reticent to to hardcode the number of previous results in the helper, since it's possible that value could change over time. If you wouldn't mind making that change, then I think this is

Also, I noticed that the spinner weirdly isn't showing up anymore, but it's unrelated to this changeset because it's also happening in staging. I'll check in on Slack before ticketing.

[jazairi]

I'd prefer this be set to @pagination[:per_page], as the actual value will vary depending on what RESULTS_PER_PAGE is set to in the Analyzer model. (We should probably make that an env var, but that's neither here nor there.)

[JPrevost]

Doh. Fully agree!

[JPrevost]

Updated, thanks for catching that! Also incorporated a change requested by Dave to use Previous 0 for the first page results.