Attachment file size limit can be set via environment variable

[caronc]

Description:

Related issue (if applicable): refs #130

Introduced new environment variable called APPRISE_ATTACH_SIZE which is defined in MB (Megabytes)and defaults to200`

The NginX configuration was updated to allow up to a maximum size of 500 MB for this value if you wish.

Checklist

• [ ] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [ ] Tests added

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.07% :warning:

Comparison is base (669619e) 94.97% compared to head (e375e9e) 94.90%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #131 +/- ## ========================================== - Coverage 94.97% 94.90% -0.07% ========================================== Files 15 15 Lines 1433 1434 +1 ========================================== Hits 1361 1361 - Misses 72 73 +1 ``` | [Files Changed](https://app.codecov.io/gh/caronc/apprise-api/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chris+Caron) | Coverage Δ | | |---|---|---| | [apprise\_api/api/utils.py](https://app.codecov.io/gh/caronc/apprise-api/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chris+Caron#diff-YXBwcmlzZV9hcGkvYXBpL3V0aWxzLnB5) | `67.72% <0.00%> (-0.31%)` | :arrow_down: |

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