Increase pip install connection timeouts

This is an attempt to reduce the number of errored builds.
Bugfix in apply_markup_filter()

lxml fails if text is empty string when applying markup filter

It is necessary to this work for the API.
API: add support Pootle API using Tastypie

* Basic API capable of listing used languages
* Add exception in CaptchaMiddleware to let pass API requests
* Include Tastypie in base requirements
* Add new settings for the API
* Docs about new settings for the API
API: add initial support for listing projects and TPs
API: add initial API docs

[ci skip]
PEP8 fixes.
Add String and Number in Javascript styleguide
Formatting fixes, small reword.
Removed prehistoric and unused middleware.
Refactored form field addition for further reuse.
Refactored for further reuse.
Created agreement form factory for better reuse.
Removed extra line.
add_page_field belongs to the form class.
Fixed typo.
Store pages and user in the agreement form.
Moved relative date calculation to PTL.common.
Removed unused import.
Track who accepted a legal document and when.

The tracking information will be required at registration time and users
will be prompted again if any of the existing agreements change or new
ones are added.

Fixes bug 2831.
Update pootle_js.po

Quick fix for bug #2857