[FEATURE] Extension upgrade script

Does the following:

* Adjusts "page template, this page" and "page template, sub page" fields to use proper extension key indication
* Adjusts FlexForm XML stored in the pages table for pages which either inherit or have selected a fluidpages_bootstrap template (note: if root line inheritance is interrupted, pages beneath the interruption page will not be detected)
Merge pull request #24 from NamelessCoder/upgradescript

[FEATURE] Extension upgrade script
[TER] 2.0.0