Add support for per feed webhook

[misthios]

Did you follow the guidelines?

• I have tested my changes
• There are no breaking changes (There is a db migration for the feed table to add a new column)
• I really tested my changes and there is no regression
• Ideally, my commit messages follow the Conventional Commits specification
• I read this document: https://miniflux.app/faq.html#pull-request

This PR adds support for webhooks on a per feed basis as well as a default webhook url which serves as a catch all: The old webhook url has been changed to act as the default webhook url which is used when there is none set for the feed. This also maintains backwards compatibility.

Renamed webhook integration settings:

New per feed webhook url field:

Solves/fixes:

• webhook integration per feed #2080

Inspired by: #2039

Please note that all the webhooks share the same secret, this was done to keep the changes minimal.

information used in tests:

• default webhook url: https://webhook.site/7878a859-d198-48fd-98f3-abe5f1e5c984
• custom url: https://webhook.site/f711867a-2124-4fd4-9ecb-c565e5dc40f2

Only default webhook url set with multiple feeds (old behaviour):

This makes 1 request per feed, since i tested with 2 feeds i got 2 messages.

Feed with custom url:

Set custom url.

Webhook output.

[fguillot]

Can you please rebase your pull-request to solve the merge conflicts. There is also some minor linter issues and missing translations that prevent the unit tests to pass.

[misthios]

I will rebase and fix the issues later today, draft until done