fix(notifications): set timestamp for sent emails

[RadovanTomik]

Negotiator pull request:

Description:

Sets timestamp during email sending.

Checklist:

Make sure you tick all the boxes bellow if they are true or do not apply:

• [x] I have performed a self review of my code
• [x] My code follows Google Java Code style
• [x] I have made my code as simple as possible
• [x] I have added unit tests and the code coverage has not decreased
• [x] I have updated the documentation in all relevant places

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.41%. Comparing base (abb755c) to head (6077fae).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #321 +/- ## ============================================ + Coverage 79.39% 79.41% +0.01% Complexity 1091 1091 ============================================ Files 147 147 Lines 3281 3284 +3 Branches 179 180 +1 ============================================ + Hits 2605 2608 +3 Misses 552 552 Partials 124 124 ``` | [Flag](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/321/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/321/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | `79.41% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC#carryforward-flags-in-the-pull-request-comment) to find out more.

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