Clarify template creation UI 

Several people were confused by the template creation UI; this change attempts to make the experience less confusing:

1. The "create directory" checkbox has been removed; the template is now in full control of this option. 
2. If a template does not require a new directory (i.e. `create_dir` is unset or false), it is no longer necessary to give a location or name for the template before creating it; instead, the template's contents are read into an unsaved editor buffer (just like other new R Markdown documents) and no changes are made on disk. 
3. If a template does require a new directory, then the directory picker UI appears with explanatory text. 

Note that this implies that a template *must* set `create_dir` if it contains more than `skeleton.Rmd`. Other heuristics are possible here (for instance, we could check for multiple files and override `create_dir` when they are present).