[multilingual/login] Persist selected language in login after logging in + Language select in menubar

[skarya22]

Brief summary of changes

• Adds language selector dropdown to all authenticated pages under my preferences
• If changed, it updates the user's preferred language
• Language selector on Login page now saves a local cookie, after logging in it gives priority to the cookie rather than the user preference
• The cookie is cleared when you logout so that it defaults to LORIS default language
• This solves two problems:
  • Changing languages when logged in was confusing as you had to open my_preferences, and then save, and exit the module to see the changes
  • Changing the language before logging in, and then logging in reverted the site to your preference, which was strange

Testing instructions (if applicable)

1. Open a LORIS instance with languages in the language table
2. On the login page, select a language that is not your user's language in my_preferences, then login and see that the language persists
3. Go to my preferences without changing anything else, see that the my_preferences is still the other language, but the site is in the language you selected
4. Hover over your account name in the menubar and change the language to something else, see that the language is changed in your user's preferences
5. log out and see that the login page is in english again
6. change the language to something else and login, and see that the site is in that language, but when you go to my preferences, it is still the language you changed it to in step 4

[racostas]

LGTM.

[racostas]

@driusan , this one is looking good to me.
Thanks !