Implemented the bolt content translations

[Spomsoree]

This feature adds the Content-Translation page:

This page allows to export content in the xlf format, which is the default format for translation agencies.

After translating the files, they can be imported again and will replace localized text with the given file.

If content cannot be found by their id (which happens if content is changed between import and exporting), the controller searches for the original text and adds the translation to the found content field.

This also works with custom fields that are stored as json.
On export the controller even splits json content, so translation agencies do not need to read in between the json overhead:

    <unit id="fZD0nJb" name="DE TITLE">
      <notes>
        <note category="id">9</note>
        <note category="type">generic</note>
        <note category="original">{"title":"DE TITLE","size":"h2"}</note>
        <note category="part">title</note>
      </notes>
      <segment>
        <source>DE TITLE</source>
        <target>EN TITLE</target>
      </segment>
    </unit>
    <unit id="_Zj_Bq_" name="h2">
      <notes>
        <note category="id">9</note>
        <note category="type">generic</note>
        <note category="original">{"title":"DE TITLE","size":"h2"}</note>
        <note category="part">size</note>
      </notes>
      <segment>
        <source>h2</source>
        <target>h2</target>
      </segment>
    </unit>

HTML on the other side is not extracted, which was not a problem for the company that translated our last project (their tool was able to do it on its own).

This was tested on one project so far (without the fallback to the content query, if id's changed).
This has also been successfully tested on all supported databases: mysql, sqlite and postgresql.

[bobdenotter]

Cool! I've not needed this functionality myself, but I can imagine it's pretty useful for a bunch of other people.

I'm out-of-town until January 2nd, so i'll do a proper review in the first week of January. :-)

[Filoz]

I really love this feature!
we had the same need

[Spomsoree]

@Filoz were you able to test it already?

Let me know if you ran into any kind of problems with that.

[Filoz]

Unfortunately not yet.
I'd like to try soon

[Spomsoree]

@Filoz did you find the time to test this?

[github-actions]

In an effort to keep our pull request list manageable, we are marking this PR as stale because it has been 60 days without activity. If this PR is still relevant, please let us know by posting a quick comment or update so that it can be assigned and hopefully merged.