Add no purge, manual delete options (#12)

- Add htmx, hx attributes
- Only add/update items since last check
- Move messages to session to persist across redirects
- Polish styles a bit (still WIP)

INSTALLING.md

@@ -46,7 +46,8 @@ The default format for dates and times look like "May 28, 2023 at 3:15pm". Chang
 
 ### Item Purging
 
-Feed Reader Central tries to keep the database tidy by purging items that have been read and are no longer required. There are three variants:
+Feed Reader Central tries to keep the database tidy by purging items that have been read and are no longer required. There are four variants:
+- `Feed::PURGE_NONE` does no purging (items have a "Delete" button, so they may be deleted manually) 
 - `Feed::PURGE_READ` purges non-bookmarked read items for a feed whenever it is refreshed. This is the most aggressive purging strategy, but it is also the only one that will not purge unread items.
 - `Feed::PURGE_BY_DAYS` purges non-bookmarked items that are older than `PURGE_NUMBER` days old. This is the default value, and `PURGE_NUMBER`'s default value is 30; items will be kept for 30 days, read or unread.
 - `Feed::PURGE_BY_COUNT` purges items to preserve at most `PURGE_NUMBER` non-bookmarked items for each feed.