Fix model ids for adding, editing, deleting attachments

[parapente]

Fixes #145

[parapente]

@JLTRY sorry it contains the fixes from PR #144 so review that first.

[JLTRY]

For AttachmentsHelper the id is quite long, prefer
$randomId = base64_encode('add' . $parent_id);
except that quite good you can merge

[JLTRY]

Also the screen for updating is quite large with a scroller (was already like it)

I modified for instance with
Pat2811.txt
I let you adjust

[parapente]

The screen seems a bit off from what it should be.

And it is the same when in full screen. There is some extra vertical space but I get no scrollbars. I have to set a very small height/width to get a scrollbar. Maybe something else is eating up the space? Some bootstrap class that you have overriden maybe?

[JLTRY]

I will check later on my side for the screen, to see why the screen it like it at my side . Let merge the change

[JLTRY]

I fixed the width of the iframe locally for my template.
For an unknow reason the width height were small
I put by default
iframe { width:100%; height:100%; }