save meta data with model archives

[epwalsh]

Changes proposed in this pull request:

• Saves meta data in the serialization directory and archive file when training. The meta data is in JSON format, and for now just includes the version used for training, but in the future could include other things such as system info or git info (save git status when run commands #4862).
• When an archive is loaded, a UserWarning will be issued if the current version of AllenNLP is older than the version that the model was trained on, or if the major versions differ (e.g. 1.2.0 vs 2.2.0) since there is no guarantee of compatibility between major releases.

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