Fixes no new missions

[kevinjtwu]

Resolves #3126

Fixes broken "no more validation missions left" screen when there are no more labels to validate. Adds different functionality for mobile, as well as translations.

Before/After screenshots (if applicable)

Before:

After:

Testing instructions

1. Force else case in getDataForValidationPages() in ValidationController.scala by temporarily editing if statement to 'if (false)'
2. Navigate to Validate webpage
3. Use Dev Tools to test mobile version

Things to check before submitting the PR

• I've written a descriptive PR title.
• I've included before/after screenshots above.
• I've asked for and included translations for any user facing text that was added or modified.
• I've tested on mobile (only needed for validation page).

[misaugstad]

Did some quick testing and found some issues!

1. When finishing a mission and there are labels left to validate, the "Validate more labels" button is now broken on both mobile and desktop! Make sure to test thoroughly!!
2. The text in the button on mobile when you're out of labels is too small
   

[misaugstad]

Thanks @kevinjtwu! I updated the CSS slightly so that the button is wider and doesn't wrap to two lines. Otherwise looks good.

I think that they CSS and stuff is really messy, but these screens aren't seen very much and really need an overhaul, so I'm not as worried about making them perfect.