Simplify rendering/editing sheet data between Vue app and backend

Branching this from #747 as it's a bit more complicated.

Developing complex features with zero design for planning has resulted in overly complex data structures in some cases. We should rethink this and simplify the data, if/where appropriate.

NOTE: If possible, this change should be implemented by making required changes to e.g. sheet editing configurations via a Django migration script, so that wiping out existing configurations would not be required.

Current Situation

There are currently multiple levels of describing nodes, columns and formatting in rendering/editing
- Object Class (obj_cls)
  - Type of node (material or object)
  - Critical for all stages of ISAtab processing
  - In some cases, might be able to gather this info without passing it explicitly in Ajax view data
- Item Type
  - Type of material node, used both in rendering and editing
  - SOURCE, SAMPLE, DATA, etc.
  - Critical for all stages of ISAtab processing
- Header Type
  - Type of field in a process/material object
  - name, protocol, process_name, field..
  - field = e.g. characteristics and other columns of which there can be 0-N for a node
- Column Type (colType or col_type)
  - Type of column, used for rendering
  - Null = default string rendering
  - Example: LINK_FILE for column which is supposed to render a link to iRODS for a DATA style material
  - Critical for rendering data and sorting columns
  - Some info can be derived from other sources
    - E.g. name in header type + DATA in item type = LINK_FILE (always)
    - However, a field may be of e.g. NUMERIC or UNIT type
  - Should at the very least be renamed into something more descriptive. Render type? Value type?
- Format
  - Used in sheet edit config, defines the format of data accepted for a column
  - For e.g. characteristics, a single column may have one of multiple types of formatting
  - For e.g. name and protocol, the format` is always the same (redundant info?)
  - Critical for headers/columns with header type of field, not sure if for other types?
Naming of headers/columns
- This may be inconsistent in places and confusing for the user
- Also should consider the ISAtab/altamISA perspective
- TODO: Catalogue how these terms are currently used

Plan for Improvement

?????? (-> profit)

Tasks

TBD (see previous list)

bihealth / sodar-server