Enable creating personal account

[gorisanson]

Enhancement suggestion from @e6p77Bi8CW7zRBg:

It would be good if a user can create a personal account so that

1. Number of wins and losses are recoded
2. User have nickname
3. Game has ranking system

[gorisanson]

Suggestion from @repeat_c, which is relevant to this issue:

Building accounts system is not that easy work, so how about showing just a part of the public IP address of the peer to prevent users from misunderstanding the identification of the peer?

[gorisanson]

I think building accounts system is not that hard if I utilize the Firebase Authentication service. But I'm worried about the management, which would be followed after building the accounts system, which is expected not that easy because of the increase of complexity of the source code and the increase of traffic from/to the Cloud Firestore, which I utilize now, or the quick match server. I want to minimize my management work required to keep running the peer-to-peer online version. So I decided to just implement the following:

1. Enable to show nickname to the peer. (It is from the suggestion 2 of @e6p77Bi8CW7zRBg.) The nickname is not under uniqueness test, just kept locally by Web Storage API. And it is communicated via the webRTC data channel so that it does not require any additional traffic to/from the Cloud Firestore or the quick match server.

2. Show a part of public IP address from the peer, so that a "loose identification" could be done. (It is the suggestion of @repeat_c.) For now, since no useful information about the peer is showed, sometimes users does mistakes by thinking that they met the same peer once more in the quick match mode.

[gorisanson]

Showing nickname and partial public IP address function is merged to branch master by the commit 2da126e.