Now only traverse images when directory exists

Problem
everytime I modified pt files in embedding_dir or hypernetwork_dir, I
need to restart webui to have the new files shown in the dropdown of
Train Tab

Solution
refactored create_refresh_button out of create_setting_component so we
can use this method to create button next to gr.Dropdowns of embedding
name and hypernetworks

Extra Modification
hypernetwork pt are now sorted in alphabetic order

revert changes to two bat files I asked to revert but the author couldn't in 863e9efc.

* moved repository url to changeable environment variable

* move stable diffusion repo itself to env

* added missing env

* Remove default URL
Co-authored-by: AUTOMATIC1111 <16777216c@gmail.com>

Default value is the first value of selectables.
Particually, None in styles.

made save style button not die when you cancel