This reverts merge request !16626

Use limit for search count queries

See merge request gitlab-org/gitlab-ce!16502

Return more consistent values for merge_status on V4 MR APIs

Closes #20639

See merge request gitlab-org/gitlab-ce!16626

To make it more clear to developers that the entity exposes the
application secret, define a separate entity that only should be used
when the secret is needed (probably only on creation).

Search query is especially slow if a user searches a generic string
which matches many records, in such case search can take tens of
seconds or time out. To speed up the search query, we search only for
first 1000 records, if there is >1000 matching records we just display
"1000+" instead of precise total count supposing that with such amount
the exact count is not so important for the user.

Because for issues even limited search was not fast enough, 2-phase
approach is used for issues: first we use simpler/faster query to get
all public issues, if this exceeds the limit, we just return the limit.
If the amount of matching results is lower than limit, we re-run more
complex search query (which includes also confidential issues).
Re-running the complex query should be fast enough in such case because the
amount of matching issues is lower than limit.

Because exact total_count is now limited, this patch also switches to
to "prev/next" pagination.

Related #40540

Resolve "Project/#/Members?query= blank causes 500 error"

Closes #41673

See merge request gitlab-org/gitlab-ce!16235