🚸(backend) use unaccented full name for user search

We have the user full name through OIDC in the database, but the search only
used the email field.
This change allows to search for a user by their first and/or
last name (fix #929).
Given that user names are more likely than emails to include diacritics, it
unaccents both the query and the database entry for search (fix #1091).
It also unaccents for email so that internationalized domain names are
managed whether or not the accent is included in the search.
An unaccented gin index is added on users full_name an email fields.
Using a manual migration because a wrapper around unaccent is necessary
to make it IMMUTABLE (cf.
https://stackoverflow.com/questions/9063402/ )

CHANGELOG.md

@@ -64,6 +64,7 @@ and this project adheres to
 - ♻️(frontend) preserve @ character when esc is pressed after typing it #1512
 - ♻️(frontend) make summary button fixed to remain visible during scroll #1581
 - ♻️(frontend) pdf embed use full width #1526
+- 🚸(backend) use unaccented full name for user search #1637
 
 ### Fixed