Closes #23460

To make design consistent with project settings

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Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Explored Policy framework to create something I can use as a starting point.

When we added caching, this meant that calling `can_be_resolved_in_ui?` didn't
always call `lines`, which meant that we didn't get the benefit of the
side-effect from that, where it forced the conflict data itself to UTF-8.

To fix that, make this explicit by separating the `raw_content` (any encoding)
from the `content` (which is either UTF-8, or an exception is raised).

Also disables occurrences highlights & the line highlight for the review
mode.

Closes #44307

Now the issuable reference check works only in project scope, if we
reference an issuable from a non-project resource (e.g. epics), then
project is not set, and there is mismatch in generated issue references.

This patch enables issuable reference state check also from group scope.

Closes gitlab-ee#4683
Related to #30916

- Change find_cluster_platform_kubernetes to ignore environment argument
- Fix broken specs

CE Port of: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5047/

Closes gitaly#1101

When the database is in a read-only state, display a banner on each
page informing the user they cannot write to that GitLab instance.

Closes gitlab-org/gitlab-ce#43937.

Closes #28857