Limited post size on MySQL backend

[blacklight]

Describe the bug

posts.content is modelled as a text column on schema.sql.

The TEXT data type has a limit of 2^16=64KB on MySQL.

This means that longer posts (longer than ~8000/9000 words in a language like English) can't be stored without having the database to throw an error.

(Note that you don't have this limitation on SQLite, where the maximum size of a data type isn't enforced and it only obeys the laws of SQLITE_MAX_LENGTH, which is usually ~1GB).

Steps to reproduce (if necessary)

Steps to reproduce the behavior:

1. Create a post with more than 64K characters on a MySQL backend
2. Submit
3. Submission will fail - a database error will be logged

Expected behavior

Posts up to a reasonable length for a blog article should be allowed on all the supported databases.

Application configuration

• Single mode or Multi-user mode?
• Database? mysql
• Open registration? N/A
• Federation enabled? N/A

Version or last commit:
N/A

Workarounds

1. If you are planning to initialize your db through : change the data type of the content column on the posts table - from text to mediumtext.
2. If you have an existing database, change the data type of posts.content:

ALTER TABLE posts MODIFY COLUMN content MEDIUMTEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL;

Proposal

Change posts.content data type from TEXT (size: 2^16 on MySQL) to MEDIUMTEXT (size: 2^24 on MySQL).

It's quite straightforward how to do it from schema.sql or via alter commands, but I'm not familiar enough with the SQL generation logic in db/create.go (I could take a look into it if there's interest in a PR though). At a first glance it seems that a new ColumnTypeMediumText needs to be added and then handled (as MEDIUMTEXT for MySQL, and simple TEXT for SQLite), but I'm not sure if this will cover everything (plus I guess that there would also be a migration logic to add).