rename current "Stateless" mode to "Ephemeral"

[ewsopp]

We are adding a new "Stateless" mode that will not use local storage at all, even temporarily as the current stateless mode does. I want to use the "Stateless" name for that new mode.

This newly named "Ephemeral" mode should remain the default mode set by the Stateless Setup Assistant (current functionality).

• rename Stateless mode to Ephemeral.
• update this mode's description from

Store and serve media files with Google Cloud Storage only. Media files are not stored locally.

to

Store and serve media files with Google Cloud Storage only. Media files are not stored locally, but local storage is used temporarily for processing and is required for certain compatibles.

[planv]

Done.
Also changed all related usage stateless mode in code, so in the future we won't have misunderstanding with mode stateless and ephemeral.

[alimuzzaman]

@planv did you wrote a migration script for the renaming of the mode?

[planv]

@alimuzzaman Yes

[alimuzzaman]

@ewsopp can we add Previously known as Stateless or something similar in front of ephemeral?

Users may get confused.