Cleaning up some warnings from ESLint

Fixes most of #477

New list is:

> $ eslint **/*.js
> 
> 1. **bin/key_server.js: line 82, col 12, Error** - noncedb is already declared in the upper scope.
> 1. **bin/mail_helper.js: line 68, col 6, Error** - mail is already declared in the upper scope.
> 1. **routes/index.js: line 5, col 4, Error** - 'crypto' is read only.
> 1. **routes/password.js: line 8, col 4, Error** - 'crypto' is read only.
> 1. **test/test_server.js: line 28, col 20, Error** - Empty block statement.
> 1. **tokens/index.js: line 5, col 4, Error** - 'crypto' is read only.
> 1. **bin/signer.js: line 13, col 6, Error** - Unexpected dangling '_' in '_privKey'.
> 1. **client/index.js: line 24, col 2, Error** - Unexpected dangling '_' in '_devices'.
> 1. **client/index.js: line 118, col 0, Error** - Unexpected dangling '_' in '_clear'.
> 1. **client/index.js: line 127, col 2, Error** - Unexpected dangling '_' in '_devices'.
> 1. **client/index.js: line 223, col 8, Error** - Unexpected dangling '_' in '_clear'.
> 1. **client/index.js: line 269, col 8, Error** - Unexpected dangling '_' in '_devices'.
> 1. **client/index.js: line 270, col 15, Error** - Unexpected dangling '_' in '_devices'.
> 1. **client/index.js: line 277, col 10, Error** - Unexpected dangling '_' in '_clear'.
> 1. **client/index.js: line 281, col 2, Error** - Unexpected dangling '_' in '_clear'.
> 1. **server/server.js: line 273, col 18, Error** - Unexpected dangling '_' in '_code'.
>
> 16 problems

We can ignore the "unexpected dangling" (7-16) and probably the rest too.