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

After selecting a group, the projects dropdown will now filter by group projects instead

Closes #29489

Fixed pipelines tabs not showing arrows

Some inner page tabs where not scrollable like the top nav & this caused
them to break onto new lines mobile.

This pages include:

- dashboard/projects
- project -> merge_requests
- search
- project -> milestones
- project -> pipelines

Closes #18853

center spinner
show placeholder text on new MR page
remove redundant `display: block` from li - they default to `list-style` already

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1765

Recudes the number of queries within the snippet search from approx. 50
to approx. 14 by preloading the authors

Part of #27392

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/24036

Also enabled syntaax highlight on the search results

Closes #26832

Apart from Issues and Merge Requests pages, there are other sort/filter
dropdowns that needed updating.

Signed-off-by: David Wagner <david@marvid.fr>

Truncato is introduced as a dependency to intelligently shorten the rendered
HTML to 200 characters; the previous approach could have resulted in invalid
HTML being rendered.

Closes #21868

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

This ensures all these calls have an author set, allowing the use of
"all" mentions where possible.

Changed some variable names
Fixed CHANGELOG entry