Deprecate twitter

We further deprecate the use of twitter.

In cases such as historic guidelines and rules, we add
a note about using Mastodon.

In cases where a tweet URL was used, we replace with
an Internet Archive link because twitter has announced
that accounts that are not used for a period of time
will be deleted.

We add a few missing deprecated_twitter information
to the author.numbers and author.cvs files.

We remove references to twitter handles (after making
sure that both author.numbers and author.cvs has the
deprecated_twitter information).  When the "_README.md_"
files have their header information generated by
a tool, such deprecated_twitter data will not be present.

2018/poikola/README.md

@@ -3,7 +3,6 @@
 Timo Poikola <ioccc2018@ig.fi>  
 <https://ig.fi/>  
 <https://ig.fi/other/ioccc>  
-Twitter: @timpoiko  
 
 ## To build: