Add Import and Export pages

[ai]

This task is too adding the core feature, without a design (I will do it myself later).

We need to support formats:

• OPML. Only for atom/rss/json-feed.

• Internal format. JSON file with feeds, posts, keys. The format will contain the full state of feeds including lastOriginId, lastPublishedAt, etc. We will use this format for debug and to backup the app state.

• [ ] Add Import and Export pages to Manage Feeds settings section.

• [ ] Add export page

  1. On first step user need to choose the format OPML/Internal.
  2. On the second for OPML user will see All feeds/Select feeds radio
  3. On selecting Select feeds user will see all feeds on the same page (by categories) with checkboxes (and checkbox for category). Also buttons to Select all and Unselect all.
  4. Then we need a loader on the file generation process
  5. And then browser will suggest user to save file
  6. If user, select internal format, we will ask him All posts/No posts, All feeds/Select feeds. Then the feeds selection if necessary.

• [ ] Add import page

  1. On the page user will see only type=file control.
  2. After selecting the file, we will ask what feeds user want to import from the file (the same all/select and list of feeds)
  3. Then we may need some loader (use starting store for now, because we will need to hide menu) for the import process

All feeds selection, import/export logic should be done in core/. In web/ we just connect stores/methods to Web API.

[sanchopanda]

I'd like to try to do this task