improve event image selection

[kernc]

currently, one selects and image from a combobox with hunderds of images for events from ages ago. basically, one needs to upload a new image every single fucking time.

instead, we'd use a thumbnail preview gallery with, further, tag filter field.

is this possibly related to #74 ?

[kernc]

also, new image upload should require not a description but image tags.

[kernc]

also, the irrelevant ancient images can be pruned from db.