Create plugin scaffold + define institution table database schema

[SteelWagstaff]

Start with pressbooks-plugin-scaffold and tear out/replace the stuff we don't want. Use PSR-12 and other tooling devs want for databases.

Define the database schema needed for creating and maintaining institution lists.

See https://github.com/pressbooks/pressbooks-multi-institution/issues/1 for requirements for institutions

DoD:

• [ ] this plugin can be installed via a bedrock and activated on a network
• [ ] when activated, the plugin creates the institution database table needed for this feature

[SteelWagstaff]

We probably want to create three tables: wp_institutions wp_institutions_sites wp_institutions_users

[ho-man-chan]

Update:

• Read up on Mantle documentation.

[SteelWagstaff]

@arzola opened this PR: https://github.com/pressbooks/pressbooks-plugin-scaffold/pull/28