Improve update mechanism

[andrewreeman]

v2.1.12 was released in which only one plugin had actually had a real update.

1. It would be useful to show update notifications only when particular plugins are updated.

The current mechanism of querying the update could be improved in general. For some reason I decided to use an url pointing to my website: https://www.andrewreeman.com/spectral_suite_publish.json
2. When of course this could point to a similar json file in the main branch on the spectral suite project.

3. Perhaps worth doing in a different issue, but consider having some way of ignoring a single update, although it can still show it on the about page. Then if there is a new update that is not ignored show the notification again.