MDL-38693 themes: amended z-indexes

Amended #dock z-index level from 11000 to 5000 across all themes; amended z-index style of #page-header in Overlay theme such that elements no longer occlude TinyMCE.
MDL-43835 phpunit: Create unique starting id's for sequences.

When all starting autoincrement ids are the same, it's difficult to detect coding and testing
errors that use the incorrect id in test calls.  The classic case is cmid vs instance id.
To reduce the chance of the coding error we start sequences at different values where possible.

OUBlog installation is the case that highlighted this as it inserts a sitewide course module
at install time and cmid <> instance id in most cases.
MDL-46013 theme: Uninstall theme unconditionally unsets configs
MDL-43835 phpunit: Correct unit tests.

When different ID's are set for each sequence a number
of unit test failures appear.  They have been corrected
to allow unit tests to pass with the new generator in place.
MDL-44124 libraries: Allow rrules with only one part during rrule validation checks
MDL-35453 quiz reports: don't order by id.

We were using ORDER BY id even though there was a perfectly good
attempt column to order the attempts by one user at one quiz.

Also, Oracle was complaining:
Debug info: ORA-01799: a column may not be outer-joined to a subquery
MDL-46086 quiz: fix unit test on Oracle
MDL-37145 caching: noted recommendation for dedicated memcache servers

Several reports have been made about the unexpected purging of entire
memcached servers when MUC is configured to use Memcache and caches
get purged.
This is expected and occurs because the memcache extenions don't
provide isolated containers. Moodle data is stored along side any other
data in the cache and when caches get purged all data gets removed
whether it belongs to Moodle or not.

The docs cover this point and here I've added notes about this to both
config-dist.php and cache/README.md
Merge branch 'MDL-44124-master' of git://github.com/ankitagarwal/moodle
Merge branch 'MDL-46013_uninstall_theme' of git://github.com/moodlerooms/moodle
Merge branch 'MDL-43835' of https://github.com/mr-russ/moodle
Merge branch 'MDL-46086' of git://github.com/timhunt/moodle
Merge branch 'MDL-35453' of git://github.com/timhunt/moodle
MDL-41746 External api: Allow multiple calls to validate_context at different levels

(Only from webservices)
Merge branch 'MDL-38693-master' of git://github.com/jethac/moodle
MDL-45580 assignfeedback_editpdf: Store a readonly version of the PDF
MDL-45580 assignfeedback_editpdf: Delete draft content on new submission
Merge branch '37145-28' of git://github.com/samhemelryk/moodle
MDL-45580 assignfeedback_editpdf: Save readonly version in backup
Merge branch 'MDL-45580-master' of git://github.com/FMCorz/moodle