of mentions when there's too many of 'em

Capitalization like that is only ever used in article/book titles, I
have not seen another website use it in interface elements.

- "Post" for post submission (we already had the button title be "Posting"
when the post was being sent, so there was a weird inconsistency)
- "Register" for registration submission
- "Save changes" for usages in settings

Also removes admin_menu.roles from some localizations since it is not
used anywhere

I did not add a translation for my native language in this patch because
I am not sure how weblate would react, but I did add it locally and it
seems to work.

move external source button to extra buttons, make expand button easier to click and highlight on hover