[Tagging] Make ObjectTag.object_id case-sensitive

[pomegranited]

Story

"As a content author, I need my content tags to reference a case-sensitive object_id, because the content object IDs are case sensitive too (e.g. an org with short name "MIT" is different from one with short name "mit")."

Description

cf openedx/openedx-core#67 (comment)

Completion criteria

• Small PR to make ObjectTag.object_id a case sensitive char field instead of case-insensitive.
• Check the other char fields for what could legitimately be made case sensitive as well.

Relevant repositories

• https://github.com/openedx/openedx-learning/