[WIP] Database changes for v005

[ErisDS]

This PR contains a partial migration. As such, it will put your database into a half-state. Only checkout this PR if your local branch is running on a database you don't care about (or have a copy of).

If you are on this branch, and switch back to master, your database will be too far ahead, and will cause the following error:

ERROR: Your database is not compatible with this version of Ghost 
 You will need to create a new database 

When this happens, you have 2 options:

• delete the database
• run FORCE_MIGRATION=true npm start to try to force the db back into a safe state (if you're on a production blog, read this)

Also, if changes are made to this PR and you've already run an earlier version of the 005 migration, you won't get the latest changes. In this case FORCE_MIGRATION=true npm start will cause the migrations to be re-run which should get your database into the right state.

refs #6301, #6165, #6255

• Adds a new mobiledoc field to posts for the new editor spike
• Adds a new is_private field to posts, so we can look at making only specific posts private
• Changes tags.hidden to tags.hashtag. Atm that makes a new field - the migration tools need updating

[ErisDS]

This has been rebased against the reorganisation branch, but is still a WIP (see #6301 (comment)).

Whilst revisiting this, I figured it would make sense to make the new mobiledoc field longer inline with #4134.

It's currently set to "long", which in MySQL has a max length of 4,294,967,295/4GB (see http://stackoverflow.com/questions/13932750/tinytext-text-mediumtext-and-longtext-maximum-storage-sizes). However the maximum field length in postgres is 1GB (see https://wiki.postgresql.org/wiki/FAQ#What_is_the_maximum_size_for_a_row.2C_a_table.2C_and_a_database.3F) and the default max length for a field in SQLite3 is also 1GB (see https://www.sqlite.org/limits.html#max_length).

Therefore, the maxlength property (which is used for validations) has been set to 1000000000, which I believe will ensure we're catering for the minimum value across the 3 databases.

Wondering if it's worth doing this now, or whether it should match the html field to start with and get increased with the others. My thinking is, the format contains more syntax, so it probably will need more space than the HTML version.

[ErisDS]

Closing in favour of #6628

I'll rewrite each of the migrations in the new style and pull them across one-by-one.