Add a common function for webhooks to convert multipart strings to dict.
This facilitates loading a multipart/form-data fixture as a file string,
and converting it.
This will allow testing integrations that use multipart/form-data,
and generating their example screenshots using a script.
Note that this only supports text fields, accommodation for binary files
is not included at the moment.
This commit relocates all the scripts in the tools directory which
are used for auto-generating screenshots to the new
tools/screenshots directory to avoid cluttering the tools/ root.
