Make memes collapsible

[dylf]

It would be nice to have the meme images collapsable like media embedded via a link in mattermost.

This is doable by not posting the link to the generated image in markdown but only posting the link as it is.
IMO this should be opt-in with a configuration switch.

The relevant line is in server/plugin.go on line 201:
replace

    Text:         "![" + template.Name + "](" + siteURL + "/plugins/memes-adsy/templates/" + template.Name + ".jpg" + queryString + ")",

with

    Text:         siteURL + "/plugins/memes-adsy/templates/" + template.Name + ".jpg" + queryString,

I started to work on it: https://github.com/aconitumnapellus/mattermost-plugin-memes/commits/feature/opt_in_preview