[WIP] add coverage map depicting the location of all images in the map

[boney-bun]

fix #536

[boney-bun]

@Maite-Machipi @lavhengwa i need your feedback for this ticket.
i'm not sure how to fit all the data in the current search window as there are more than a million data.
so i propose to show all the data by opening a clean map with only basic navigation.

below is the illustration of what i meant:

what do you think?
if agreed, i'll continue to populate the map with all data.

[boney-bun]

update on showing the whole data at once:

i encounter a potential problem while trying to obtain the whole data from the server.
the server will likely be crashed when we request the 1 million data all at once.
the illustration above works with only 100 data.
so, i'm looking at the possibility to use spatial clustering to ease the problem of getting a lot of data from the server.

what do you think @lavhengwa @Maite-Machipi @gubuntu ?