Move untranslated strings into data-i18n attributes

[rebecca-shoptaw]

Closes #8848.

Fix. Makes string translation possible by internationalizing and moving text from JavaScript to HTML.

Technical

Very simple and easy to replicate in similar PR's! Of the various data-i18n examples in the codebase, the one I liked most and went with involves just:

1. Adding i18n strings to the top of the HTML file, i.e.:

$ i18n_strings = {
    $ "empty_excerpt": _("Please provide an excerpt."),
    $ "over_wordcount": _("That excerpt is too long.")
    $ } 

2. Adding the strings as an attribute to the div where the text will be displayed: data-i18n="$json_encode(i18n_strings)"
3. In the JavaScript, replacing the plain string with a call to the data-i18n attribute:

const i18nStrings = JSON.parse(document.querySelector('#excerpts-errors').dataset.i18n);

if (!data.excerpt) {
    return error('#excerpts-errors', '#excerpts-excerpt', i18nStrings['empty_excerpt']);
}

4. Regenerating the messages.pot file using the relevant Docker command to keep everything up to date

Note: There's a currently un-used string -- "Please enter a valid URL." -- added to the data-i18n for web.html to prep for that string's addition as part of #8871.

Testing

Previously untranslated strings like "Please choose an image or provide a URL" should appear in the messages.pot file.

Stakeholders

@jimchamp

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 16.00%. Comparing base (42f396d) to head (6037e0a).
Report is 6 commits behind head on master.

Files	Patch %	Lines
openlibrary/plugins/openlibrary/js/edit.js	0.00%	6 Missing ⚠️
openlibrary/plugins/openlibrary/js/covers.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8874      +/-   ##
==========================================
- Coverage   16.02%   16.00%   -0.03%     
==========================================
  Files          89       89              
  Lines        4693     4699       +6     
  Branches      818      820       +2     
==========================================
  Hits          752      752              
- Misses       3434     3439       +5     
- Partials      507      508       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jimchamp]

Thanks @rebecca-shoptaw --- lgtm!