Remove language selection from the general configuration

refs #6073
Update preference form so that it suits the new language negotiation

refs #6073
Update web bootstrapping to suit the new localization procedure

Icinga\Application\Web::setupInternationalization now sets up the locale
from the current session first, if any, and falls back to the one from the
user's preferences, if any.

refs #6073
Remove autotools files

In preparation for package w/ web installer installations all autotools files must be removed
because the installation method using the latter will no longer be supported.

refs #7505
tests: remove extcmd_test

extcmd_test is no longer used anywhere because I removed all command pipe tests.
Remove files related to API doc

I remove these files because the script for creating the API was removed too.
But we'll re-add them sometime I guess.
Clean .gitignore file

Note to all developers: You must not add your local crap here.
Use .git/info/exclude.
Do not spread .gitattributes over multiple files
Remove config/*.in files
Remove example config files memberships, permissions and restrictions
Add standard authentication.ini
Merge branch 'master' into feature/localization-form-6073

Conflicts:
	application/forms/Config/GeneralForm.php
	application/forms/Preference/GeneralForm.php
	library/Icinga/Application/Web.php
Add new `section` column to icingaweb_user_preference table
Add new table columns and change the preferences DB save process
Add translation to preference tab title
Remove timezone element from the ApplicationConfigForm
Add timezone detection to `setupTimezone' in ApplicationBootstrap
Fix `setupTimezone' and `setupInternationalization' for user preferences
Fix IniStore to save and load proper user preferences
Change PreferenceForm and add the ability to store preferences in the current session