Add HTTP Headers option

[ev-gor]

Closes #6784

Changes proposed in this pull request:

• 'HTTP Headers' field was added to 'Add' and 'Update' subscription forms
• Values from this field are saved by feed/subscription controller
• A title and a hint for the field were added to lang files

How to test the feature manually:

1. Open 'Add' form for a new feed or 'Update' form for an existing one.
2. Add your HTTP headers to 'HTTP headers' field.
3. Save the form and update the feed.

Pull request checklist:

• clear commit messages
• code manually tested

[Alkarex]

There is an encoding issue to fix. Looking at it

[Alkarex]

Let me address the encoding issue in another PR, as it affects a few other fields as well.
Thanks, and please add a line for you in https://github.com/FreshRSS/FreshRSS/blob/edge/CREDITS.md

[Alkarex]

@ev-gor Could you please test #6821 with special characters such as <'&"> ?

[ev-gor]

@Alkarex Yes, it works.

Before a server received a header:
authorization: 'Bearer <TOKEN>&'some'&"other"'

Now:
authorization: 'Bearer <Token>&'some'&"other"'

[Alkarex]

Excellent. Thanks for the feedback. Additional tests on all the other fields also welcome.

[Nikkiiw]

how to add a referer？
thx！

[Frenzie]

Under advanced, like this:

Referer: https://example.com/page?q=123

(The typo-looking spelling of referer is intentional)

[Alkarex]

We could add a Referer example in the help box underneath. PR welcome