Capitalise Twitter, GitHub; Remove dupe dashboard.synchronize key

- Should not need to have dashboard.synchronize key twice
- Rewords delete_account.are_you_sure as "check" is the action
  and should instead list the reason

Based on #391 and #399 already being merged
Merge pull request #400 from randomecho/copyedit-locale-en-capitals

Fix Twitter, GitHub capitalisation in en.yml and remove dupe dashboard.synchronize key