Add button "Mark as up-to-date" to AMOS Translator for translation contributors

This issue has an overlap with MDLSITE-5572, but I still create a separate issue as I do not see a browser-specific issue.

The AMOS translator in its current version lacks a "Mark as up-to-date" button just as admins have it in the Moodle language customization.

Steps to reproduce:

• Log into AMOS
• Pick an arbitrary plugin and language pack from which you know that an english string has changed recently but which does not need a translation update. This especially applies to typo fixes, grammar fixes or other minor language improvements while the translated string is already perfectly correct
• Check the 'missing and outdated strings only' option
• Click 'Search strings'

Expected result:

• In the right column, there is a "Mark as up-to-date" button which lets you mark a string as up-to-date
• When you submit the changes to the maintainer and when the maintainer approves the changes, the string is marked as up-to-date
• The next time you visit AMOS translator, this string is not listed in the 'missing and outdated strings only' filter anymore and is shown with a green background.

Actual result:

• In the right column, there isn't a "Mark as up-to-date" button
• I also don't see any other possibility to mark a string as up-to-date. I just can make a minor modification to the translation to turn the string to a blue background and then revert this change.
• When I submit such changes to the maintainer, this often does not have any effect.
• The next time I visit AMOS translator, this string is still listed in the 'missing and outdated strings only' filter and is shown with a yellow background.

With regard to MDLSITE-5572, I do not see any browser-specific issue. The problem appears for me in Firefox and Chrome and I also do not see any buttons or widgets in the HTML code: