adding omnivore saveurl integration

[jhjaggars]

Do you follow the guidelines?

• I have tested my changes
• I read this document: https://miniflux.app/faq.html#pull-request

This PR introduces a "save" integration for Omnivore. In order to use it you must generate an API key and input it into the integration section of the settings.

This PR depends on the google/uuid package (which is already an indirect dependency).

[fguillot]

--- FAIL: TestMissingTranslations (0.01s)
    catalog_test.go:86: Translation key "form.integration.omnivore_api_key" not found in language "de_DE"

https://miniflux.app/docs/i18n.html#update-translations

[fguillot]

There is no error shown in the logs if the API key or the API URL is incorrect. It should at least check the response given by Omnivore API.

[jhjaggars]

I've addressed this by following the pattern in the pocket integration (returning an error for http response codes >=400). The response body is logged as well which might be helpful, but I was unsure if the risk of dumping a ton of text into the log is worth it. Let me know if you'd like to limit or completely omit this part.

[fguillot]

When testing, Omnivore seems to return a JSON response like this regardless of the error:

error="omnivore: failed to save URL: status=500 {\"errors\":[{\"message\":\"Unexpected server error\"}]}\n"

It would be nice to parse the JSON response, and log only the error message. It could be done in another pull-request.