Start implementation of jobtypes API
Disallow specifying jobs on creating jobtypes

This is strictly a read-only field.
Implement SingleJobTypeAPI
Implement delete method for single jobtypes
Fixed some docstring mistakes
Implemented jobtype/code endpoint

This allows clients to directly fetch the python code of a jobtype.
Bugfix in single jobtype/get

For some reason, SQLAlchemy sometimes returns the code column of JobType
as bytes instead of a string.  If this happens, convert it to string.
Minor docstring fix
Implement PUT method for single jobtype/code
Implement API for jobype software requirements
Decode bytes only in Python 3
Fix some docstrings

[ci skip]
Make jobtype deletion cascade to its versions
JobTypeSoftwareRequirement ref -> jobtype versions

With the changes towards versioning jobtypes. jobtype software
requirements need to reference a specific version of a jobtype instead
of just a jobtype.
Make to_dict() unpack jobtype_version relations
Change jobtype api to reflect jobtype versioning