add a modal with hint about how to choose pass phrase

[Kharchevskyi]

There should be a little question mark icon at the end of pass phrase creation prompt texts like Choose your pass phrase (?). When clicked, this shows:

[tomholub]

@ekievsky do you have any android device handy? Our Android app is further along in implementation, and a lot of iOS functionality can be copied from there. It will be helpful to you to be able to use the app as an inspiration when implementing issues.

[Kharchevskyi]

@ekievsky please make sure that #323 is merged or checkout from feature/issue-197-pass-phrase-memory.

[tomholub]

I've put this issue into 0.2.1 milestone which is to be done after all issues in 0.2.0 are done - therefore the above should already be merged by then.