Simplify the entry adding interfaces in `DataStore`

[mylibrar]

Is your feature request related to a problem? Please describe.
This is follow-up issue from #834 (comment). Right now DataStore keeps a long list of entry adding methods:

• add_annotation_raw
• add_audio_annotation_raw
• add_image_annotation_raw
• add_link_raw
• add_group_raw
• ...

This might be cumbersome for both users and developers. DataStore already provides a _is_subclass method and there is no need to register a different method for each top level entry. We should try to merge them into add_entry_raw and simplify the interface.

Describe the solution you'd like

• Design the interface. Now that we are merging all the entry adding methods, we should take care of the different input arguments. For example, add_link_raw accepts begin_tid and end_tid while add_group_raw takes in a member_type. The merged function should account for all these differences.
• Implement the new function. The basic structure might a long list of if...elif...elif...else with each branch conditioning on DataStore._is_subclass.
• Update all the references. Since add_***_raw() is already invoked in a few places in codebase, we need to update these references.

Describe alternatives you've considered
Notes:

• Some arguments like tid is common to all add_***_raw() methods while others (e.g., allow_duplicate) may only apply to annotation-like entries.
• Actually, many _new_***() methods (e.g., _new_link, _new_annotation, _new_group, etc.) are also pretty similar. We might consider merging them as well.

Additional context

• This is part of the data efficiency project
• This PR should be made to the master branch.