* remove useless is_string check
* use the DEFAULT_LANGUAGE when Accept-Language is invalid.

Reimplement the __() and __f() functions using Symfony.

ci: some cleanup of the gitlab-ci file and testing migrations on MySQL

See merge request framasoft/framadate/framadate!388

This commit splits the `test` job into three separate jobs:
- test
- lint
- db:migrate

It also factorise some code, and remove useless use of git checkout

Update pot after !387

See merge request framasoft/framadate/framadate!392

(also make sure it's pushed to Zanata)

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

i18n: localize the date format placeholders

See merge request framasoft/framadate/framadate!387

Add development SMTP server in docker-compose

See merge request framasoft/framadate/framadate!391

For instance, when using French, display the date input placeholder
as "jj/mm/aaaa" (instead of "dd/mm/yyyy").

This doesn't change the way dates are actually parsed – only what is
displayed to the user changes.

Add locales for !385

See merge request framasoft/framadate/framadate!386

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

locales: fix missing "Expiry date" key

See merge request framasoft/framadate/framadate!385

Extract mail content creation and sending into the NotificationService

See merge request framasoft/framadate/framadate!382

Fix date format

See merge request framasoft/framadate/framadate!381

Parsing and displaying

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Fix already selected days not being highlighted

Closes #361

See merge request framasoft/framadate/framadate!380

Workaround the datepicker appearing below the add_days modal on Safari

See merge request framasoft/framadate/framadate!379

scripts: fix translations

See merge request framasoft/framadate/framadate!377

The correct solution would probably be to upgrade the version of bootstrap and bootstrap-datepicker that we use

My understandng of the bug is:
* bootstrap-datepicker finds the first ancestor whose z-index isn’t auto, and sets its own z-index to the found elemenents z-index + 10.
* On Firefox and Chrome, the modal element is returned (its z-index is 1050)
* On Safari, the modal-dialog element is found. Its z-index is auto, but Safari reports this as '0', which makes the z-index of the datepicker 10.

Translations with a comma in the key are rewritten to be on a single
line.

But this failed when the comma was just preceded by a dot (like in
`etc.,`).

Fix the script so that it works even in those cases.

Update locales

See merge request framasoft/framadate/framadate!376