Shareable URL with filters

[niconoe]

We are now propagating URLs such as:

https://alert.riparias.be/?filters=%7B%27speciesIds%27%3A+%5B32%5D%7D

(see https://www.riparias.be/353 for example)

That works, but there are a few things that could be improved:

• No tests to make sure this feature is not deleted/broken
• No documentation to tell users they can use them
• The filter encoding in URL is quite ugly to humans (beware of existing links if we decide to change the scheme!)

[niconoe]

After some search, it seems the current format is appropriate (not great for human readability, but easy to encode/decode and we don't have many alternatives).

It would still be good to test and document it, though.

[damianooldoni]

Ok, Nico.

I agree then on adding documentation for getting such links.

[niconoe]

This has now been documented in CONTRIBUTING.md