MDL-39248 Fix the plugins check screen strings

Not all add-ons require administrator's attention during the upgrade
now. They are now listed only if there is an update available for them.

Also, it's not true any more that add-ons must be updated manually. They
can be updated using the automatic deployment feature, too.
MDL-39249 Return to the correct page after successful update deployment

The method available_update_deployer::make_execution_widget() used to
have hard-coded return URL. Now it accepts the return URL as the second
parameter and passes it to the mdeploy.php utility.

The callerurl parameter is now correctly passed and used.
MDL-39249 Fix typo in available_update_deployer class

I have accidentally spotted this typo while working on this branch. It
has never caused any harm as both parameters callerurl and returnurl are
always passed. Blame copy/pasting of code blocks.
MDL-38898 theme_bootstrap: Made changes to layout/general.php to enable drag-n-drop support in Bootstrap theme.
MDL-36959 rework adding of content files to the file pool

This patch includes refreshing of borked files in file pool and basic prevention of race conditions. It also helps with diagnosing of file pool permission problems, detects coding errors and some other type of problems including sha1 collision jackpot.
MDL-36959 test reworked adding of files to sha1 content pool
MDL-39178 add missing update callback for all theme settings
MDL-39213 allow role assignments in frontpage in user upload
MDL-38874 theme_bootstrap: added back CSS rules to fix advanced elements show/hide toggle in admin forms.

While doing this I got a compile error so not sure but I am guessing the changes to fix that error will show up in the commit diff.
Merge branch 'MDL-39241-master' of git://github.com/danpoltawski/moodle
MDL-39196 theme_bootstrap: improved display of login errors
Merge branch 'MDL-38874_master' of git://github.com/lazydaisy/moodle

Conflicts:
	theme/bootstrap/style/generated.css
MDL-38874 theme_bootstrap: tidy up whitespace
Merge branch 'MDL-39172' of git://github.com/jmvedrine/moodle
Merge branch 'wip-MDL-38900-master' of git://github.com/ds125v/moodle

Conflicts:
	theme/bootstrap/style/generated.css
Merge branch 'MDL-39220' of git://github.com/colchambers/moodle
Merge branch 'MDL-38854_master' of git://github.com/bmbrands/moodle

Conflicts:
	theme/bootstrap/style/generated.css
Automatically generated installer lang files
MDL-39118 - mod_data: removed areamaxbytes option for uploading files and images.

Setting the maximum size for a file or picture field to the course upload limit
would send a value of 0 to the file picker. This patch checks for this 0 value
and handles it appropriately.
Merge branch 'wip-MDL-39118-master' of git://github.com/abgreeve/moodle