Disallow sync on read-only sphere contexts

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Currently we check most write operations to verify that the author has read-write access to the sphere context before proceeding. Although there is no danger that the author can create valid sphere history when these writes are allowed, the check enables us to surface an appropriate error / warning to the writer that they attempted a write that would not be allowed.

We do not currently do this check on sync, even though sync may change the local sphere history (effectively a write). This has come up at least once when someone tried to sync a sphere that was traversed to (so definitely was read-only) and got an obtuse error from Noosphere.

So, we should perform this check for read-write access before proceeding with a sync.