MDL-36874 mod_book: Add uarrow() output renderer.

In preparation for Book navigation improvements in MDL-36874, this commit
adds a uarrow() output renderer, to complement rarrow() and larrow().
MDL-36874 mod_book: Add text navigation.

Based on a patch by Matthew Cannings, this replaces Book's previous/next arrow
image links with a choice of: no links, image links, or chapter title text links.
MDL-48041 messages: Removed HTML table around user images
MDL-48146 quiz settings: fix non-percent feedback boundaries
MDL-47700 Grades: Sitewide overview report isn't visible in some cases

Showing a overview report rank sitewide isn't visible to student in same cases
MDL-47700 Grades: Fix incorrect config for overview report showrank
MDL-47434 format_weeks: section 0 navigation tweak

Section 0 is now only shown in the navigation block if it contains
activities.
If there are no activities then the section is removed.
This mimicks the display on the course page.
MDL-47434 format_topics: section 0 navigation tweak

Section 0 is now only shown in the navigation block if it contains
activities.
If there are no activities then the section is removed.
This mimicks the display on the course page.
MDL-33606 course: Adding section info to course_viewed log

Also changing references to section id to
section number, that it is what it really is.
MDL-47800 auth: Add option to logout on pw change
MDL-47803 Add report that lists user sessions
MDL-47587 Lesson: Display progress bar on the end of lesson page
Merge branch 'MDL-47700-master' of git://github.com/xow/moodle
Merge branch 'wip_MDL-47803_m28_usersessions2' of https://github.com/skodak/moodle
Merge branch 'MDL-48146' of git://github.com/timhunt/moodle
Merge branch 'm28_MDL-48041' of https://github.com/totara/moodle

Conflicts:
	theme/bootstrapbase/style/moodle.css
Merge branch '47434-29-1' of git://github.com/samhemelryk/moodle
Merge branch 'MDL36874_Text_navigation_for_Books' of git://github.com/Dave-B/moodle

Conflicts:
	mod/book/version.php
	mod/book/db/upgrade.php
Merge branch 'wip_MDL-47800_m28_pwchange2' of git://github.com/skodak/moodle
MDL-47800 admin: improved strings