"Cash by Mail" instructions improvements

[Jakub-CZ]

1) Fix spaces in "Cash by Mail"

In the English string portfolio.pending.step2_buyer.cashByMail a space is missing after the 2nd sentence.

Screenshot in Transifex:

2) Clarify the process of mediation

@pazza83's suggestion from #5092 (comment)

[ghost]

Hopefully when the revised trading rules text is ready, @Jakub-CZ will include it in this PR.
Relevant discussion of the proposed change: #5092 (comment) and onward.

[Jakub-CZ]

As suggested by @jmacxx, I've implemented @pazza83's suggestion from #5092 (after the PR got merged).

Additional changes:

• fixed wording of "... could lead to lose both trading and deposit funds" (in the 5th bullet point in the second list; please somebody review);
• made the popup wider for fewer line breaks;
• bumped up the hardcoded character limit to avoid ellipsis in this long block of text.

[ripcurlx]

utACK