Remove the buttons form the image dialoge WIP

[allcaps]

Hello @thibaudcolas,

AltText does reset on close/open. Needs work.

[thibaudcolas]

Thank you @allcaps. Sorry it took quite a while for me to get back to this. I wanted to get a new release out with bug fixes before looking into enhancements.

The main reason why we didn't manage to get this working at the sprint is because... there is a bug in Draft.js. We were changing the image entity data but it wasn't reflecting in the UI, because at the moment entity data is a mutable global store and updating that only will fail to trigger a re-render of anything in the <Editor> component, which (wrongly) assumes its input to be immutable.

Long story short, I fixed that by adding a "useless" change on block data.

See facebookarchive/draft-js#839 for further details.

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I also changed something somewhat unrelated – in the past, properties altText and alt were used interchangeably to handle the img's alt. This is now using alt everywhere.

[thibaudcolas]

Coverage decrease is because I forgot to ignore a file in master. Will fix that there.