Rework API filtering logic

[rhigman]

          Overall, rather than `TimeExpression` I'm more inclined to support filtering by field more widely. Something like:

pub enum Operator {
    Eq,
    Neq,
    Gt,
    Lt,
    Gte,
    Lte,
    Ilike,
}

pub struct WorkFilter {
    pub field: WorkField,
    pub operator: Operator,
    pub value: String,
}

fn works (
        [...]
        filters: Vec<WorkFilter>,
        [...]
)

Originally posted by @ja573 in #484 (review)

To do:

• replace filter, filter_param_1, filter_param_2 and filter_param_3 in Crud::all() and Crud::count() with a single filters param, as above
• move existing field-specific filter logic from crud.rs modules into graphql/model.rs, continuing to expose most of the existing field-specific filter params to the user for compatibility/ease of use
• treat all expressions that share the same field as OR statements

[rhigman]

OR statements

The existing filter logic creates two kinds of OR statements:

1. Same field, multiple values [same operator]: e.g. filter_param_1 might be used to check for works whose workType equals monograph OR edited_book (this case was covered in the original design above)
2. Same value, multiple fields [same operator]: e.g. filter might be used to check for works whose title OR abstract contains keyword

[rhigman]

In progress at https://github.com/thoth-pub/thoth/tree/feature/486_rework_filtering. Basic proof of concept achieved for statement type 2 above. Design work still needed on how to also handle type 1. Currently on hold.