A filter's context (previously referred to as its `kind`) controls where
the filter is applied.
This was implemented as an enum with a specific property to control how
it would serialise when @FormUrlEncoded, and with a @Json annotation for
Moshi.
In addition, the model objects kept the filter context in its string
form throughout Pachli, requiring periodic conversion to/from the enum
type, making the code more complicated.
Fix this, by:
1. Converting the incoming JSON value to the enum type immediately, so
the rest of the code uses the enum constants exclusively.
2. Implement a Retrofit converter that serialises the enum value when
@FormUrlEncoded to the same string used in JSON serialisation
9a23439d04