refactor notification sending; dedicated notifier that can run standalone or integrated; add mail limit

[notifications] return 400 instead of 500 if token does not expire and a calendar notification is requested

[notifications] remove events for revoked (or removed) mytokens from a calendar before the ics is returned

Bumps [github.com/gofiber/fiber/v2](https://github.com/gofiber/fiber) from 2.48.0 to 2.49.1.
- [Release notes](https://github.com/gofiber/fiber/releases)
- [Commits](https://github.com/gofiber/fiber/compare/v2.48.0...v2.49.1

)

---
updated-dependencies:
- dependency-name: github.com/gofiber/fiber/v2
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [golang.org/x/term](https://github.com/golang/term) from 0.11.0 to 0.12.0.
- [Commits](https://github.com/golang/term/compare/v0.11.0...v0.12.0

)

---
updated-dependencies:
- dependency-name: golang.org/x/term
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [github.com/jinzhu/copier](https://github.com/jinzhu/copier) from 0.3.5 to 0.4.0.
- [Commits](https://github.com/jinzhu/copier/compare/v0.3.5...v0.4.0

)

---
updated-dependencies:
- dependency-name: github.com/jinzhu/copier
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [github.com/redis/go-redis/v9](https://github.com/redis/go-redis) from 9.0.5 to 9.1.0.
- [Release notes](https://github.com/redis/go-redis/releases)
- [Changelog](https://github.com/redis/go-redis/blob/master/CHANGELOG.md)
- [Commits](https://github.com/redis/go-redis/compare/v9.0.5...v9.1.0

)

---
updated-dependencies:
- dependency-name: github.com/redis/go-redis/v9
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>