Add feed-level bulk actions (download | reset | delete)

[shooftie]

Sometimes it would be nice to add a podcast without downloading all of the episodes at once (because my 'incoming' drive is full). It would therefore be nice to have some controls to perform batch actions:

• download all: to download any episodes that have not yet been downloaded
• _redownload all/re-sync files: to re-sync the files with the database i.e. download any podcasts that the db thinks should be there but aren't e.g. perhaps because the original file is missing but the DB doesn't know
• re-sync db: to re-sync the database with the files i.e. update the UI to represent changes to the filesystem e.g. the file has been removed from the filesystem but is not represented in the UI
• delete all: to 'reset' the podcast back to the original state. This is essentially just reimplementation of the "manage podcasts" UI but on the individual podcast page.