Refuse to execute if binary permission allow non-user write

[chikeichan]

According to specs, Clef UI SHOULD verify the permissions on the signer binary, and refuse to execute or warn if permissions allow non-user write.

For completion of this task, Clef UI should:

• Verify permission on signer binary
• If the permission allow non-user write, display a message that says "Your signer binary allows non-user write. Clef will not start"
• We should show the same button that allow the user to set their own directory to geth.