Refactor DOIPlugin to only flash success messages in blueprint views

[MarkCalvert]

The reason behind this change is to handle when CKAN background jobs update datasets (e.g. embargo releases) which trigger the after_dataset_update interface.
Flash messages will fail in background jobs and only work from web requests.
The solution is to check if the request has come from a blueprint view.

[alycejenni]

Hi @MarkCalvert, thanks for your PR!

• trying to access toolkit.g is breaking the tests (specifically test_after_dataset_update in test_plugin.py)
• if possible, it would be great if you could create a test that would check this setup (the background job)
• please make sure you read the contributing guidelines for commits for your new commits

[MarkCalvert]

Hi @alycejenni ,
Sorry I was aware of that issue a while ago and forgot to merge the commit into this PR branch.
d1b65dc
I have run the tests locally and they now pass.