Add "post privacy" selection for boosts

[trzyglow]

Pitch

A simple optional popup that asks which "post privacy" a boosted post should be boosted with.

Here's an example of the feature as implemented in the web/desktop version of Mastodon:

Motivation

As someone with a "private/locked" Mastodon account, the boost feature becomes partially unusable, because the default setting for boosts and posts is always "followers only"; which wouldn't be a problem if the boost button had a popup menu to change the privacy mode.

tldr;
can't boost publicly on mobile with locked account.