Add an option to require unique app names

[scottmmjackson]

Our app name generator allows an operation mode wherein unique app names are required:

https://github.com/rstudio/rsconnect/blob/main/R/title.R#L32

However, every call site for this function in deployApp passes "FALSE" to that parameter.

We should add an option to permit changing this behavior such that non-unique names produce deployment errors.

[hadley]

If we're automatically generating the app name, shouldn't we always make it unique? i.e. is there some reason we need to make this a user facing argument?

[aronatkins]

The only remaining exception is bookdown.org, which uses the content name to automatically construct its vanity URL. See bookdown::publish_book

[hadley]

Hmmm, I think different parts of rsconnect make different assumptions about what should happen if there's an existing deployment with the current name:

• generateAppName() assumes we want to create a new deployment.
• deployApp() assumes that we want to reuse an existing deployment if it exists.

I think we need to pick one of these two principles and stick to it, otherwise the package is going to inevitably be inconsistent.

(Personally, I'd be in favour of assuming we want to update an existing deployment, and minimise as much of the appName magic as possible in favour of giving the user a clear error message and forcing them to pick their own appName.)

[hadley]

And it occurs to me that the default appName should probably use appPrimaryDoc if set. Currently if you're publishing a bunch of individual documents from a common directory, they'll all have the same default appName.

[hadley]

Oooh, but it does, implicitly, because generateAppName() is passed recordPath, not appDir.