thumbnail disappears after editing metadata

[gubuntu]

reported by @timlinux during Maputo workshop

child of #457

[NyakudyaA]

Tested this and thumbnail stays visible even after editing metadata.

However, the legend disappears as illustrated below:

After editing metadata

Clicking on the legend icon to access the url results in the following:

[boney-bun]

Hi @NyakudyaA
could you test it again?
this issue seems to be no longer a problem.

[gubuntu]

seems to be fixed. although the healthsites layer on testing did not have a thumbnail to start with. Setting it did generate a new one. I then edited its metadata and the thumbnail remained