Fix to #829

[agbcix]

[BF] Unconditionally re-initialize sFlow collector counters - fixes inex/IXP-Manager#829

Previously the counters were not reset in case the API was unreachable. After writing data to RRDs new counters need to be initialized unconditionally.

In addition to the above, I have:

• [x] ensured all relevant template output is escaped to avoid XSS attached with <?= $t->ee( $data ) ?> or equivalent.
• [x] ensured appropriate checks against user privilege / resources accessed
• [x] API calls (particular for add/edit/delete/toggle) are not implemented with GET and use CSRF tokens to avoid CSRF attacks

[nickhilliard]

I've had a look at this, but fixing this properly is likely to be more complicated than this patch, e.g. startup considerations, etc. I'll have another look before working out what the best approach is