Enhancement: Variables correctifs

[znicholasbrown]

This PR:

• fixes an issue where create and edit modals had incorrect save/create/close ordering
• adds a check for value length (max 255 chars)
• ensures update is emitted across nested components (so any consumers can subscribe and refresh their data when a change has taken place)
• adds key to the VariableMenu instantiation in the table to work around an issue in prefect-design (I don't want to tackle that issue as part of this since that could have broader impact)
• truncates name and value columns

table before:

table after:

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[znicholasbrown]

Closes: #1300
Closes: #1301

[znicholasbrown]

Addressed your comments @pleek91 (some were no longer valid once I removed the in-component validation method); I've opened up a ticket for overhauling these validation utilities as they're a bit tough to keep track of atm and I don't want to address them in this PR