helpers: fix ampersands (&) breaking file templating because it is a special sed char (issues#2470)

[fflorent]

The problem

Fixes YunoHost/issues#2470

Solution

Escape ampersands (&) with \\& so it is treated as a special character. Quoting sed manpage:

s/regexp/replacement/
Attempt to match regexp against the pattern space. If successful, replace that portion matched with replacement. The replacement may contain the special character & to refer to that portion of
the pattern space which matched, and the special escapes \1 through \9 to refer to the corresponding matching sub-expressions in the regexp.

PR Status

Ready to be reviewed, the test pass.

How to test

$ bash test_helpers.sh 2
$ bash test_helpers.sh 2.1