Detect if local storage needs to be cleared

[hofstef]

Is your feature request related to a problem? Please describe.
When new versions of Egon are deployed to the egon.io website, the application seems broken to users that have used a previous version or the latest build version. The reason is that some information in the browser's local storage is outdated. Users need to clear their browser's local storage to properly update to a new version. This is documented in the user guide, but users might not be aware of that and think that Egon is broken.

Describe the solution you'd like
On loading Egon, it should check if its versiondiffers from the last used version. If that is the case, the user should be prompted to clear the local storage. A link to the User Guide should be included.

Describe alternatives you've considered
Clearing the local storage is an alternative, but has the disadvantage that the user might loose data. Also, it makes testing the compatibility of versions more difficult for developers.

[hofstef]

Released with v2.1.0