Adds new mailbox tag to Obstacle label type

[misaugstad]

Resolves #3552

Adds a 'mailbox' tag to the Obstacle label type. Here is the example image I used for our tooltips:

Unfortunately, we've now used all 26 letters of the alphabet for keyboard shortcuts, so we've moved on to special characters :sob:

Before/After screenshots (if applicable)

Before

After

Things to check before submitting the PR

• [x] I've written a descriptive PR title.
• [x] I've added/updated comments for large or confusing blocks of code.
• [x] I've included before/after screenshots above.
• [x] I've asked for and included translations for any user facing text that was added or modified.