[ticket/12699] Replace magic numbers for the TAB and for ENTER keydown

Replaced the magic numbers 9 and 13 in the keydown for the text editor
with constants in the existing keymap for phpBB.

PHPBB3-12699
Merge pull request #2610 from brunoais/ticket/12699

[Ticket/12699] Remove magic numbers in the message textarea keydown callback

* brunoais/ticket/12699:
  [ticket/12699] Replace magic numbers for the TAB and for ENTER keydown
  [ticket/12699] Add tab key to keymap
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12699] Replace magic numbers for the TAB and for ENTER keydown
  [ticket/12699] Add tab key to keymap