adds migration to convert compressed backup data to json

[yulolimum]

Resolves #129

@jamonholmgren I manually tested this on a few local sites and on the example apps. I have no idea how to test migrations programatically. You may have to help me on this.

Edit: Updated description here

[jamonholmgren]

@yulolimum Luckily, the worst that can happen is that a few older content revisions are lost. What's your confidence level that this code works?

[yulolimum]

@jamonholmgren, the code works. I do however want to add a friendly error message for people that upgrade but do not run migrations and try to restore. if that happen, itll restore a binary bitstring that will most likely break the JS. not sure if its worth it due to the low amount of sites running the backups release but still could be better ux. we can remove this error message a few major releases later.

[jamonholmgren]

Good idea, let's add that.

[yulolimum]

@jamonholmgren @ryanlntn @zberkom

This PR is finally feature complete. I went through the code and annotated a ton since the PR is pretty big.

The PR adds:

• migration to convert compressed page backups (binary) to a decompressed string - conversation here
• notifications feature - allows us to notify users of various things as well as allows devs to push custom messages to various parts of Thesis (README screenshot)
• fixes #148, #147, #129, #128

Note, as stated previously, testing migrations is kind of hard. I ran the code across a few sites and it converts everything perfectly without issues. Our site as an example had 250 backups and it took a couple seconds to convert.

Screenshots:

[jamonholmgren]

~There are conflicts on this.~ NVM, it's good. Ready to go. ✅