docs: use parallel builds to speedup docs generation

Using parallel builds to increase doc regeneration as well as speedup
Travis builds.

With hot filesystem speed are as follows depending on the -j value:

Processes	Duration
1	7.75s
3	5.2s
5	4.21s
8	3.88s
10	3.96s
15	3.38s
20	3.48s

Cold (12 runs)
1	20.27 (17.82-22.71)
15	17.91 (14.41-21.42)

Not as big a saving as I'd hoped for a cold start but good for doc
development.
Travis: install lxml without compile optimisations

Use CFLAGS="-O0" as recommended by
http://lxml.de/installation.html#installation to speedup installation
times in Travis builds.

This is applied to all installations here, should maybe be limited to
lxml.

The next step wpuld be to create precompiled wheel packages to speedup
setup.

Without this change we take on average 2,5min to setup the packages for
testing. After this we shave off 1,3min i.e. setup takes on average 57s
Cleanup: no need for extra parameter
User: Inherit PermissionsMixin for our custom User model

AbstractBaseUser, which we inherit from, does not provide "has_perm". This is
taken care of by PermissionsMixin which only AbstractUser has by default.

Incidentally this gives us back is_superuser so we can get rid of it from our
class. No migration needed.
Keeping in mind it *also* brings us groups, which is not so great since we
don't actually use those anywhere.

Fixes #3286
Migrations: Fix another dependency
Migrations: Fix frozen models and complete_apps

This allows to create migrations again using:

schemamigration accounts --auto
Migrations: Alter migration to depend on migration 0002 from accounts
Migrations: accounts.User now inherits from PermissionsMixin

Followup for 4d678d8.
Migrations: Drop table for voting app

Followup for c977e615.
Migrations cleanup: Align frozen models to ease comparison
Migrations: Fix field in frozen models across migrations
Migrations: Add dependency
Migrations: Add another dependency
Migrations: Add another new dependency
Migrations: Add missing frozen models on migration
Migrations: Fix frozen models
Migrations: Fix more frozen models

Adding dependencies on other migrations allows to simplify the frozen
models. Also all the references to PootleProfile were changed in
favor of accouns.User. Other fields have been added or removed as
necessary.
Migrations: Permissions need to be adjusted

The permissions for pootle_app.Suggestion do use the contenttype for
such model, and since we are removing that model and its contenttype,
these references need to be properly adjusted.