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-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-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
MDL-46013 theme: Uninstall theme unconditionally unsets configs
Merge branch 'MDL-35453_26' of git://github.com/timhunt/moodle into MOODLE_26_STABLE
MDL-41746 External api: Allow multiple calls to validate_context at different levels

(Only from webservices)
Merge branch 'MDL-38693-26' of git://github.com/jethac/moodle into MOODLE_26_STABLE
Merge branch '37145-26' of git://github.com/samhemelryk/moodle into MOODLE_26_STABLE
MDL-45580 assignfeedback_editpdf: Store a readonly version of the PDF
MDL-45580 assignfeedback_editpdf: Delete draft content on new submission
MDL-45580 assignfeedback_editpdf: Save readonly version in backup
Merge branch 'MDL-45580-26' of git://github.com/FMCorz/moodle into MOODLE_26_STABLE