I want to see a feed in reader without adding it

[lemon24]

I want to see any feed in reader, so I can browse before actually adding it.

If the CLI ever gets an option to show a specific feed, it should also be possible to see a feed that's not added.

---

More than one way of doing this:

1. Create a temporary reader and update it; use it to show a list of entries with all buttons disabled ("preview mode"); the only possible action is to add the feed to the main reader. This is the simplest way of doing it, since it only uses stuff that already exists.
2. (2.1) Have a way to mark feeds as temporary; alternatively, (2.2) have a way to copy a feed from one reader to another (we may do this anyway in the future as a sort of import functionality). This allows for a fully interactive "preview mode", but seems like a huge overkill.

For both of these we can do the update_feed() call:

1. As part of the view; simplest to implement, but ties up a web worker while the request happens; for 2.2 above, we'd still need a way of keeping the temporary feed around between requests.
2. As a background job; would need some sort of job queue, which requires a lot of infrastructure the app doesn't have at the moment.

1 and 1 seems like the way to go.

[lemon24]

Similar to #150 in that it shouldn't affect the Reader API.

[lemon24]

To make this work with #150, the "add feed" workflow will look like this:

1. user adds feed or page URL using the "add feed" control at the top of the page
2. redirect to the preview page

• if the URL is a feed, a preview is shown, along with an "add feed" button
• if the URL is not a feed, try to extract the links to feeds contained in the page (Extract feed link from page source #150); for each found feed, show a link to preview it

3. user clicks on the "add feed" button on the preview page
4. redirect to the newly added feed (Adding a feed should redirect to the feed page #119)