feat: postmessage handlers to view reports and load comparisons

[b0ink]

Description

Allows the JPlag app to be embedded in an iframe or opened as popup from within another web application, and automatically load report files, search for comparisons, and open a comparison if a single result is found

Examples of using postMessage from external web application

Loading .jplag report files:

this.jplagIframe.nativeElement.contentWindow?.postMessage(
  {
    type: 'upload-jplag-file',
    file: response.body, // Blob file of the .jplag file
    name: 'report.jplag',
  },
  '*',
);

Setting the search filter and auto viewing the first comparison:

this.jplagIframe.nativeElement.contentWindow?.postMessage(
  {
    type: 'set-search-filter-value',
    filter: studentUsername, // Search filter
    autoViewComparison: true
  },
  '*',
);