This reverts merge request !2992

This greatly improves the performance of Project#external_issue_tracker
by moving most of the fields queried in Ruby to the database and letting
the database handle all logic. Prior to this change the process of
finding an external issue tracker was along the lines of the following:

1. Load all project services into memory.
2. Reduce the list to only services where "issue_tracker?" returns true
3. Reduce the list from step 2 to service where "default?" returns false
4. Find the first service where "activated?" returns true

This has to two big problems:

1. Loading all services into memory only to reduce the list down to a
   single item later on is a waste of memory (and slow timing wise).
2. Calling Array#select followed by Array#reject followed by Array#find
   allocates extra objects when this really isn't needed.

To work around this the following service fields have been moved to the
database (instead of being hardcoded):

* category
* default

This in turn means we can get the external issue tracker using the
following query:

    SELECT *
    FROM services
    WHERE active IS TRUE
    AND default IS FALSE
    AND category = 'issue_tracker'
    AND project_id = XXX
    LIMIT 1

This coupled with memoizing the result (just as before this commit)
greatly reduces the time it takes for Project#external_issue_tracker to
complete. The exact reduction depends on one's environment, but locally
the execution time is reduced from roughly 230 ms to only 2 ms (= a
reduction of almost 180x).

Fixes gitlab-org/gitlab-ce#10771

Fix #9963 reference_filter "Encoding::CompatibilityError" bug with some complex URL;

https://github.com/gitlabhq/gitlabhq/pull/9964

@DouweM

See merge request !2383

Per this issue: https://gitlab.com/esr/irker/issues/2

A documentation update was added to irkerd (https://gitlab.com/esr/irker/commit/190808c37d4ab5f0f16fe35352ff36863c2732d5) but the bug is still there. Making a note of it here could save someone a lot of hassle. 

This could probably be worded better if someone else wants to take a stab at it.

ref: https://github.com/ruby/ruby/commit/238b979f1789f95262a267d8df6239806f2859cc

Do not call API if there is no API URL

Fixes #5878 

CE users may not be interested in the new JIRA features. In this case,
we should detect they haven't set an API URL and fallback to the
behavior pre-8.3. This patch does that very easily.

There are planned improvements to JIRA in future releases such as
gitlab-org/gitlab-ce#5541 which will make this more configurable.

See merge request !2341

Suppress e-mails on failed builds if allow_failure is set

Every time I push to GitLab, I get > 2 emails saying a spec failed when I don't care about the benchmarks and others that have `allow_failure` set to `true`.

@ayufan mentioned creating a summary e-mail to prevent getting one e-mail per build, but the latter might actually be desirable. For example, I do want to know if Rubocop errors fail right away.

See merge request !2178