Added show_alert boolean field to DataImport model. It defaults to true.

This attribute controls whether an alert related to OPML data import is displayed to the user in the start page or not.
DataImport model now accepts "NONE" as a valid state. This is now the default state when creating a new DataImport instance.

Until now, when a user has never ran an OPML import there is no DataImport instance in the DB for that user. With the changes being implemented in this and the next few commits, all users will have an associated DataImport record, which will have state "NONE" for those users who have not ran an OPML import.
All users now have an associated DataImport instance as soon as they are created, with status "NONE".

The view that generates JSON for the DataImports no longer has to make a special case for when the user has no DataImport, this should never happen.

Added db migration to create missing DataImports for those users that have not yet ran an OPML import.

WIP: some tests currently failing.
Fixed unit test.
Updated DataImport factory definition for tests (the default DataImport status is "NONE" now). Fixed acceptance test (it was failing because of issues with tests order).
WIP: fix acceptance tests.
Fixed unit test.
Made DataImportController#show more resistant to failures. Fixed acceptance tests.
Large images in entries have a 1em top and bottom margin. Small images in entries have no top or bottom margins, but small 3px right and left margins instead.

The reason is that small images are sometimes in the middle of a line of text, or they have their text description right after the image. In these case it looks bad if those small images have a bottom margin, because the image is one line above the surrounding text which looks weird. It's generally better that small images have no bottom/top margins. A small right/left margin is desirable in case the image itself has no white around it, to separate it from the surrounding text.
Added close buttons to data-import status notices in the start page. These buttons are non-functional for the moment.
Added a DataImportsController#update method and route.

It will be invoked when the user clicks on the "x" (close) button of the data-import-status alert. It will set the "show_alert" attribute of the DataImport instance to false, which will cause the alert to be hidden from that moment on (this is not yet implemented).
Finished: The DataImportsController#update action changes the visibility of the data_import alert, depending on the value of the "show_alert" parameter.
WIP: the import status alert in the start page can be permanently hidden by clicking on the close button (looks like an X).
When the DataImport instance for the current user has the show_alert flag set to false, the view does not display the alert related to OPML imports.

This change has required adding the show_alert flag to the set of values that are returned in the JSON for the DataImport.
The DataImport alert in the start page can now be permanently dismissed when the import status is ERROR or SUCCESS.
The DataImport alert in the start page cannot be dismissed while the import is running.
Added tooltip to the close button in the DataImport alert displayed in the start page.