- Add support for picture HTML tag
- Add support for srcset, media, and sizes attributes to img and source tags

Also includes few database improvements:

- Speed up entries clean up with an index and a goroutine
- Avoid the accumulation of enclosures for some feeds

YouTube doesn't expose RSS links anymore for new-style URLs.

In the vast majority of cases, the default entry title is correct.

Ignoring <media:title> avoid overriding the default title if they are different.

I came across a few blogs that didn't have a link rel alternate
but offered a RSS/Atom feed.
This aims at solving this issue for "well known" feed urls, since
these urls are often the same.

The default rule results in blank content.

The default rule does not show the comic posted to the feed. The comic image is in a div with id "comic".

RayWenderlich.com is a popular developer's community for iOS and Android developers. The default rule results in "GROUP GROUP GROUP GROUP…" instead of the content posted on the blog.

Fixes #617

- Embed Invidious player for invidio.us feeds
- Add new rewrite rule to use Invidious player for Youtube feeds

By default fetching original content for wdwnt.com results in a snippet of the comments section, this rule captures the article content.

Some feeds are using invalid URLs (random text).

- Make sure query strings parameters are encoded
- As opposed to the standard library, do not append equal sign
for query parameters with empty value
- Strip URL fragments like Web browsers

- Add support for Google Play XML namespace
- Improve existing iTunes namespace implementation

Show comments URL for Atom feeds as per RFC 4685.
See https://tools.ietf.org/html/rfc4685#section-4

Note that only the first link with type "text/html" is taken into consideration.

This change should reduce "illegal character code" XML errors.