feat: async content download when creating via api

[fmartingr]

Added an async field to POST /api/bookmark to allow the fetching of the bookmarks content to be asynchronous. The bookmarks updated content (thumbnail and so) will only be shown in the frontend after a refresh.

Along with that, the extension API call has been left untouched since I wasn't able to test it. We need to fix #269 before improving the webextension support.

Fixes: #302
Blocked by: #367

[phiatlux]

Hi @fmartingr . Thank you for you work here. The async task is a major time-saver (at least for me).

It seems to me though, that the POST /api/bookmarks/ has a minor issue with its content field.
Although the rest of the elements (images, thumbnails, archives etc.) are being populated asynchronously, as expected, and they do appear after I refresh the home page, the content (html field) remains empty.
POST /api/bookmarks/ext doesn't have any issues. It fetches the data and populates all the fields correctly.

[fmartingr]

It seems to me though, that the POST /api/bookmarks/ has a minor issue with its content field.
Although the rest of the elements (images, thumbnails, archives etc.) are being populated asynchronously, as expected, and they do appear after I refresh the home page, the content (html field) remains empty.
POST /api/bookmarks/ext doesn't have any issues. It fetches the data and populates all the fields correctly.

Hey @phiatlux thanks for reporting this! Since I mix and match the extension and the website I haven't caught that, I just assumed some sites had shitty HTML hierarchy and contect couldn't be extracted. I've opened #406 and will get on it ASAP.