Version 0.5.2

Hotfix since 0.5.2 release candiate:
 * tickets: fix bug that allow index of other users

Pull request #167 from azul/feature/i18n-for-ticket-system:
 * fix flash for creating anonymous tickets
 * adopt tests to new translations
 * destroy_btn helper method
 * move users key into layouts scope so it does not conflict with users
   scope
 * add btn helper for link_to with .btn
 * remove icon_color variable - yagni
 * sorting translation keys some
 * navigation works with empty locale selected
 * tickets: structure i18n
 * flash_for with_errors option displays error messages
 * remove unused bold helper and instead sanitize flash
 * Controller#flash_for instead of FlashResponder
 * split up and refactor TicketController#update
 * separate tests for the ticket list from main controller test
 * splitting up long functional test case
 * move comment related tests out of TicketControllerTest
 * use i18n.missing_translations

Pull request #168 from azul/bugfix/fix-login-validations:
 * clearify identity validations
 * ensure User#reload returns self
 * hand on errors from Email to Identity to User
 * catch corner cases of account creation
 * adopt tests to new error messages for identities
 * allow changing the user_id on an identity
 * ensure identity is cleared on user.reload - fixes test
 * use Identity for testing login availability

Pull request #163 from azul/feature/3398-save-hashed-token
 * hash token with sha512 against timing attacs #3398

Pull request #165 from azul/feature/cert-fingerprints
 * change from GET to POST for certs
 * store fingerprints with timestamp
 * store cert fingerprint with main user identity
 * SmtpCertsController, routes and tests
 * fix Email so User.new.valid? does not crash
 * basic integration test for cert API
 * calculate cert fingerprints to store for leap_mx

Pull request #166 from elijh/feature/footer
 * better detection if price link should be shown in the footer

Pull request #162 from azul/feature/3295-custom-error-pages
 * little bit of documentation
 * i18n for error pages
 * custom error pages for 404 and 500 errors