Update external_docs.rst

Added "Pootle Tutorial: Guide for translators and developers" for Pootle 2.5.1
refresh_stats: Do not break when checking last_submission in a fresh store

Fixes #3230
Requirements: Stable TTK 1.12.0 is out
editor: RTL fixes
Depcheck: Use the right constant to check our minimum required lxml
Get right count for next goal translate action

This fixes #3226
Avoid unnecessary calculations when retrieving just regular goals

And also simplify a bit the code.
Order goals by priority and by name

This way we get a prioritized list in the breadcrumbs goal dropdown.
Cleanup: There is no #summary to update, so remove unnecessary code
Bugfix: Show stats for projects with dots in its code

The JavaScript code was replacing some characters in the project code (see
https://github.com/translate/pootle/blob/fc615e8e/pootle/static/js/stats.js#L132)
for retrieving the HTML nodes to set the counts, but the HTML output by
Python didn't had those characters replaced, and thus it was impossible
to locate such nodes.

Fixes #3246