User selectable time zones

[microstudi]

Currently, it is possible to define the organization time zone for the organization.

We want to add this options at the user level too.

Mockup:

Criteria:

• Add a migration for the User model, similar as the organization with the personal timezone config there.
• Add a form entry in the "My account" where to choose the time zone. Default value must be the same as the organization.
• Modify decidim-core/app/controllers/concerns/decidim/use_organization_time_zone.rb so it uses the organization time zone or the user timezone accordingly.
• Add some tests

References:
This PR introduced timezones in the organizations:
decidim#5607

Note: Create the PR from the branch release/0.26-stable against release/0.26-stable in this repository

[antopalidi]

I made changes to the code.
But I have doubts about the third point. And for him I did not make a test.