Applying the snake_case convention throughout the repo (#967)

* Applying the snake_case convention throughout the repo

- Also removing the file extension where not needed
- Adding {{ }} on a few old pages

* Addressing concerns

- Added {{ }} where they were missed out
- Removed spaces inside {{ }}
- Reverting "file" to "filename" to make it clearer

* Fixing the comments on nc page

pages/common/electrum.md

@@ -4,11 +4,11 @@
 
 - Create a new wallet:
 
-`electrum -w {{new-wallet.dat}} create`
+`electrum -w {{new_wallet.dat}} create`
 
 - Restore an existing wallet from seed offline:
 
-`electrum -w {{recovery-wallet.dat}} restore -o`
+`electrum -w {{recovery_wallet.dat}} restore -o`
 
 - Create a signed transaction offline: