WIP: Modify entry summaries before saving them in the database so that:

- all links open in a new tab (target="_blank")
- summaries are sanitized, removing any scripts or similar and leaving only text, links and images.
Merge branch 'master' into links-new-tab
The manipulation of the entry summary necessary for links to open in a new tab will be performed in a callback at the Entry model, instead of being part of the FeedClient.fetch logic. Deleted changes that were made to the fetch logic.

Also, entry summaries are already being sanitized (twice, at the model and view levels). No need to do any changes for this.
All links in entry summaries are opened in a new tab when the user clicks them.

To achieve this, a target="_blank" attribute is added to all such links before saving the entry in the database, replacing any previous target= attribute they could have.

Also if a user tries to subscribe to a feed he's already subscribed to, besides showing a notice message he's shown the Start page.