Refactor named parameters feature to allow extensibility

[ipamo]

Refactor parsing of (and access to) named parameters into a single extensible Parameter class. The feature remains the same but the implementation is cleaner.

In addition, the actual Parameter class used by the library may be changed using a setting named DASHBOARD_PARAMETER_CLASS. This allows users of the library to implement custom parameter parsings without changing the default behaviour of the libary.

See issue #149 for extension ideas (%(value)d, %(flag:true)b, etc).

Note: I changed the id of the input HTML fields (from numerical qpX to qp_{parameterName}) so that these fields may be generated directly from instances of the Parameter class.

All tests pass.

[simonw]

This is really neat, thank you.

[simonw]

I'm afraid I'm going to have to revert this change back out again - I couldn't get the test suite to pass with it, and when I manually tested I got errors like this one:

I tracked that down to a KeyError being thrown in this block of code:

django-sql-dashboard/django_sql_dashboard/views.py

Lines 244 to 257 in 486736f

	try:
	cursor.execute("BEGIN;")
	start = time.perf_counter()
	# Running a SELECT prevents future SET TRANSACTION READ WRITE:
	cursor.execute("SELECT 1;")
	cursor.fetchall()
	PARAMETER_CLASS.execute(cursor, sql, parameters)
	try:
	rows = list(cursor.fetchmany(row_limit + 1))
	except ProgrammingError as e:
	rows = [{"statusmessage": str(cursor.statusmessage)}]
	duration_ms = (time.perf_counter() - start) * 1000.0
	except Exception as e:
	query_results.append(dict(base_error_result, error=str(e)))