[feature/revisions] Initial schema setup

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[feature/revisions] Schema for develop/create_schema_files.php

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[feature/revisions] Add local moderator permission

PHPBB3-10755
[feature/revisions] Add config setting to ACP Post Settings

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[feature/revisions] Added constant for revisions table

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[feature/revisions] Fixed typo in constants.php (thanks t_backoff!)

PHPBB3-10755
[feature/revisions] Fixed schema, save revision on edit

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[feature/revisions] Added revision-related objects and page.

I create a page called revisions.php because I could not decide on
a better place to put this. viewtopic.php is the next best if we
prefer that instead.

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[feature/revisions] Add user forum permission for viewing own revisions

I also renamed the core _class to _post since that is more accurate.

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[feature/revisions] Load current revisions in viewtopic, add revisions.php

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[feature/revisions] Serve revision-specific data from revisions table

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[feature/revisions] Add post_wiki to schema to allow wiki-style posts

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[feature/revisions] Count all revisions for total number of edits

I originally had it count only revisions including and prior to the
current revision, but it makes more sense to count all of them.

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[feature/revisions] Change last edited message

Now it displays more accurate information about that post's revisions

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[feature/revisions] Added settings for wiki-style posts

Added two forum permissions for users to create and edit wiki-style posts
Added a setting to enable wiki-style posts, and a setting for a separate
maximum number of revisions for wiki-style posts from normal posts

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[feature/revisions] Display diff between revisions

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[feature/revisions] Rework revision comparison

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[feature/revisions] all posts have >= 1 revision, put edit count on posts table

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[feature/revisions] New revision diff handling class, more work on diff

I was trying to find a good way to display the diff when I realized it
might work to do something similar to the GitHub Compare View. It's getting
late, so I'll leave off here for now, but tomorrow I plan on restructuring
it to look more like that.

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[feature/revisions] Rework logic for revision comparison

The revision compare page is being structure similar to the GitHub Compare View

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