Fixes #17075: Import plugins should be able to set last_updated field to a date in the past
Remove reference to ERROR_TWITTER_NO_CURL_EXT from language files

Causes SYSTEM NOTICE: 'Use of undefined constant' errors when using a
language other than English.

Follow-up on commit 5c5ee47bd0d853607f30ccefda8614109abda209
Change copyright year
Change revision number to 1.3-1

Publican 3.1.5 requires the revision number to match regex
'^([0-9.]*)-([0-9.]*)$' so with the current revnumber of 0 the build
errors.
Fix typo in Gianluca's name
Change book's author to "MantisBT Development Team"

Individual contributors will be credited in revision history.
Remove deprecated docbook tag

We don't have a publication number anyway
Add book abstract (replace default text)
Bug tracker, not Issue tracker
Add MantisBT logo

Replace the Publican default one
Updated revision history
Direct link to documentation page on mantisbt.org
Revise IRC section, add missing link to channel archives
Remove mention of contacting team for paid support
Revise version numbering convention section