Remove 'volta activate' and 'volta deactivate'

[charlespierce]

Related to #222
Closes #216
Closes #165
Closes #104
Closes #99

Info

• As part of the push towards automatic updates, we want to enable Volta to work directly on the command line, instead of through a shell function wrapper.
• The purpose of the function wrapper was to allow the volta activate and volta deactivate commands to work as expected, modifying the PATH environment variable for the current shell.
• However, those commands were intended as a release valve for when we were in early testing phases, and are no longer needed since Volta is much more stable now.

Changes

• Removed the activate and deactivate commands from the Volta CLI and the associated ActivityType
• Moved the VOLTA_UNSAFE_GLOBAL environment variable definition to the run modules, where it is used.
• Removed the shell and env modules, which previous existed primarily to support the Postscript behavior that is no longer needed without activate / deactivate
• Removed System::enabled_path, which was only used by volta activate before.
• Deleted tests that referenced no-longer-available blocks / commands.

Tested

• Verified that volta activate and volta deactivate are no longer allowed commands
• All automated tests pass

Notes

• This PR targets a feature branch (volta_update), as many of the changes to support automatic updates are interconnected, but can be completed / reviewed separately. The feature branch will allow that work to be reviewed in smaller chunks, as opposed to all at once.

[charlespierce]

Closing in favor of using a feature flag for updates, instead of a feature branch

[charlespierce]

Reopening as on further consideration I prefer the feature branch approach at the moment, since it allows for more parallel development and easier diffs.