Fix: WYSIWYG now returns valid HTML in tables

The WYSIWYG by default creates invalid HTML5. Creating a table and running it through W3C validator returns the following error: The border attribute on the table element is obsolete. Use CSS instead.
Use Config nesting in `SapphireTest::useTestTheme()`

Using the Config nesting functionality to restore variables
Update modeladmin.md

Corrected many broken link-renderings. Also removed the "Related" link to [genericviews Module](http://silverstripe.org/generic-views-module) because it doesn't work, and I can't it elswhere.
Update i18n.md

Fix a link being inside backticks, which broke the formatting.
Merge pull request #3171 from ss23/patch-4

Update i18n.md
Update datamodel.md

Fix broken link to SS_List->column
Merge pull request #3172 from stevie-mayhew/patch-1

Update datamodel.md
Update forms.md with additional code in MyForm.ss

Show two ways of reaching a nested field plus that field's validation message
Merge pull request #3219 from AntonyThorpe/patch-1

Update forms.md with additional example code in MyForm.ss
Fixes to the :::ss parsing - extra tabs were breaking it.
Merge pull request #3223 from camfindlay/patch-6

Fixes to the :::ss parsing tutorial 1.
Better support for overloading start and destroy methods in Session

Move functionality from static start and destroy functions into instance
methods, allowing these to be overloaded. This works the same way as
calling Session::set() which then in turn calls inst_set()

Additionally use Injector to create the default Session instance to
allow the class to be swapped out.
Update javascript.md

Replaced deprecated jQuery live() with on(), updated link to documentation.
Merge pull request #3228 from daslicht/patch-1

Update javascript.md
Merge pull request #3216 from halkyon/session_inst

Better support for overloading start and destroy methods in Session
Adding some more commonly used SS_Log priority constants.
Merge pull request #3225 from halkyon/log_constants

Adding some more commonly used SS_Log priority constants.
Update i18n.php

Correct common_languages for Norwegian to use 'nb' and not 'no'
Update i18n.php

Norwegian should use 'nb' and not 'no' in common languages array
Merge pull request #3230 from tscole/patch-1

Update i18n.php