Stabilize entry IDs

Resolves the problem of incorrectly duplicated entries in feeds that
update content but don’t explicitly provide entry IDs. Example feed:

  - https://www.to-rss.xyz/wikipedia/current_events/

Example entry:

    <item>
      <title>Current events: 2022-07-13</title>
      <link>https://en.wikipedia.org/wiki/Portal:Current_events/2022_July_13</link>
      <description>[VARIABLE CONTENT]</description>
      <pubDate>Wed, 13 Jul 2022 00:00:00 -0000</pubDate>
      </item>
    <item>

This behavior is suggested by the common practice of using an entry’s
link as its ID value, and is consistent with typical feed aggregators
such as Feedbin and Inoreader.

Closes #31

Closes #29
Fixes #24

Co-authored-by: noantiq <timucin.boldt@udo.edu>