tools (common/template): new `fallback` settings: `fallback` can be used to change fallback behaviour. By default, fallback is done on `default` theme, this can be set to an other theme with a string, or to a list of fallback. The list can also be empty is no fallback is desired (notably usefull for "main" themes, on which other themes may fallback). This setting replaces `css_default_fallback`.

[DEFAULT]
xmpp_domain = server1.test
email_server = sat.test
email_port = 8025
email_admins_list = admin@server1.test

[component file_sharing]
tls_certificate = /usr/share/sat/certificates/server1.test.pem
tls_private_key = /usr/share/sat/certificates/server1.test-key.pem
http_upload_public_facing_url = https://sat.test:8888