Resolve "Disable SAML and Bitbucket if OmniAuth is disabled"

Closes #48932

See merge request gitlab-org/gitlab-ce!20608

Resolve "Ability to download single file"

Closes #23705

See merge request gitlab-org/gitlab-ce!20480

We also try to unify the way we setup OmniAuth, and how we check
if it's enabled or not.

By default, all sessions are given the same expiration time configured in the
session store (e.g. 1 week). However, unauthenticated users can generate a lot
of sessions, primarily for CSRF verification. It makes sense to reduce the TTL
for unauthenticated to something much lower than the default (e.g. 1 hour) to
limit Redis memory. In addition, Rails creates a new session after login,
so the short TTL doesn't even need to be extended.

Closes #48101

* upstream/master: (467 commits)
  Update docs board features tier
  Upgrade grape-path-helpers to 1.0.6
  Remove healthchecks from prometheus endpoint
  Fix find_branch call sites
  Ensure Encoding.default_external is set to UTF-8 when running QA scenarios
  i18n: externalize strings from 'app/views/admin/groups'
  Backport mr widget changes from EE
  Allow to toggle notifications for issues due soon
  Vuex test helper improvements
  whitespace
  Make more ref RPC's mandatory
  Resolve "Improve performance of MR Changes tab: reduce event listeners on scroll event"
  Remove old service architecture from Vue docs
  Adding spec to test basic forking functionalities
  Fix performance problem of accessing tag list for projects api endpoints
  typo
  Add sleep to QA test before installing tiller
  Include Vue files that are not covered by tests in test coverage
  Remove Repository#path memoization
  Resolve "do not set updated_at when creating note"
  ...

Add manifest import

See merge request gitlab-org/gitlab-ce!20304