Use NotesFinder to get Noteable

See merge request gitlab-org/gitlab-ce!28205

Fix missing iid query on NotesFinder

Changed parameters of find_noteable, 
so changes across a few files were needed.
MergeRequest also requires iid instead of id query

Make NotesFinder fail with RecordNotFound again

Add specs for target_iid

Using RSpec tablesyntax for target_iid specs

Revert "Using RSpec tablesyntax for target_iid specs"

This reverts commit ba45c7f569a.

Allow find_by! here

Fix variable name

Add readable check

Revert "Add readable check"

This reverts commit 9e3a1a7aa39.

Remove unnecessary assignment

Add required changes for EE

Fix parameter count

Reduce code duplication by extracting a noteable module method

The call to find_noteable was redundant so
multiple files and lines have changed in that
commit to use the newly introduced module
method `noteable`.

Replace casecmp with include check

Add parent_type parameter


Revert "Reduce code duplication by extracting
a noteable module method"

This reverts commit 8c0923babff16.

Method is no longer needed

Check whether noteable can be read by user

Enable Get and Update of ci_default_git_depth for
Project API.

Renaming Project#default_git_depth to :ci_default_git_depth
to give more context through the API usage.

Add API documentation

Signed-off-by: Rémy Coutable <remy@rymai.me>